Python Training (3 of 4): Introductory Data Visualisation

Upcoming workshop(s) available!

The next workshop is on Tue Nov 18 at 09:30 AM.

Alternatively, check our calendar for future events.

In this third workshop we will cover

Simple visualisations with seaborn
Making modifications with matplotlib

Setting up

Spyder version

Before we begin, please check which version of Spyder you’re using (you can see this in the Anaconda Navigator, or in Help > About Spyder). If it’s less than 6, you should update Spyder before continuing. However, if you’re in a workshop, this will take too long - instead, use the following workaround if your plots don’t work:

plt.show()

Modules and data

We’ll need three modules today:

import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns

For this workshop we’ll be working from the “Players2024.csv” dataset. If you don’t have it yet,

Download the dataset.
Create a folder in in the same location as your script called “data”.
Save the dataset there.

We should then bring it in with pandas:

df = pd.read_csv("data/Players2024.csv")

Take a quick peak at the dataset to remind yourself

print(df)

                        name  birth_date  height_cm   positions nationality  \
0               James Milner  1986-01-04      175.0    Midfield     England   
1        Anastasios Tsokanis  1991-05-02      176.0    Midfield      Greece   
2              Jonas Hofmann  1992-07-14      176.0    Midfield     Germany   
3                 Pepe Reina  1982-08-31      188.0  Goalkeeper       Spain   
4              Lionel Carole  1991-04-12      180.0    Defender      France   
...                      ...         ...        ...         ...         ...   
5930  Oleksandr Pshenychnyuk  2006-05-01      180.0    Midfield     Ukraine   
5931            Alex Marques  2005-10-23      186.0    Defender    Portugal   
5932             Tomás Silva  2006-05-25      175.0    Defender    Portugal   
5933             Fábio Sambú  2007-09-06      180.0      Attack    Portugal   
5934          Hakim Sulemana  2005-02-19      164.0      Attack       Ghana   

      age                                    club  
0      38  Brighton and Hove Albion Football Club  
1      33        Volou Neos Podosferikos Syllogos  
2      32             Bayer 04 Leverkusen Fußball  
3      42                             Calcio Como  
4      33                      Kayserispor Kulübü  
...   ...                                     ...  
5930   18              ZAO FK Chornomorets Odessa  
5931   18                  Boavista Futebol Clube  
5932   18                  Boavista Futebol Clube  
5933   17                  Boavista Futebol Clube  
5934   19                    Randers Fodbold Club  

[5935 rows x 7 columns]

Seaborn for simple visualisations

To begin our visualisations, we’ll use the package seaborn, which allows you to quickly whip up decent graphs.

Seaborn has three plotting functions

sns.catplot(...) # for categorical plotting, e.g. bar plots, box plots etc.
sns.relplot(...) # for relational plotting, e.g. line plots, scatter plots
sns.displot(...) # for distributions, e.g. histograms

We’ll begin with the first.

It’s called “seaborn” as a reference to fictional character Sam Seaborn, whose initials are “sns”.

Categorical plots

Categorical plots are produced with seaborn’s sns.catplot() function. There are two key pieces of information to pass:

The data
The variables

Let’s see if there’s a relationship between the players’ heights and positions, by placing their positions on the \(x\) axis and heights on the \(y\).

sns.catplot(data = df, x = "positions", y = "height_cm")

Our first graph! This is called a swarm plot; it’s like a scatter plot for categorical variables.

If your plots don’t appear…

…AND you don’t have an error, then you might have a Spyder version with a bug.

The simplest workaround is to run plt.show() every time you make a plot. If you have time, you should update Spyder.

It’s already revealed two things to us about the data:

There are some incorrect heights - nobody is shorter than 25cm!
Someone’s position is “missing”

Let’s get rid of these with the data analysis techniques from last session

# Remove missing position
df = df[df["positions"] != "Missing"]

# Ensure reasonable heights
df = df[df["height_cm"] > 100]

Run the plot again, it’s more reasonable now

sns.catplot(data = df, x = "positions", y = "height_cm")

Bar plots

Swarm plots are interesting but not standard. You can change the plot type with the kind parameter

sns.catplot(data = df, x = "positions", y = "height_cm", kind = "bar")

Many aspects of your plot can be adjusted by sending in additional parameters and is where seaborn excels.

It seems like goalkeepers are taller, but not by much. Let’s look at the standard deviation for each position by changing the estimator = parameter (default is mean)

sns.catplot(data = df, x = "positions", y = "height_cm", kind = "bar", estimator = "std")

Clearly there’s a lot less variation in goalkeepers - they’re all tall.

Detour - line length

Notice that our last line was longer than 79 characters? That’s bad Python, and hard to read. We can fix this by making it a multi-line function, placing arguments on new lines, according to PEP 8

sns.catplot(data = df, x = "positions", y = "height_cm", kind = "bar", 
            estimator = "std")

Box plots

Let’s make box plots instead. It’s the same procedure, just change to kind = "box" and remove estimator =

sns.catplot(data = df, x = "positions", y = "height_cm", kind = "box")

Just as we predicted.

Distributions

Histograms

Let’s move to the “Age” parameter now. We can look at the distribution of ages with

sns.displot(data = df, x = "age")

Looks a bit funny with those gaps - let’s change the number of bins with bins = 28

sns.displot(data = df, x = "age", bins = 28)

Now, what if you wanted to look at the distribution for different variables? We can make a separate distribution for each position with the col = "positions" argument, specifying a new row for each position

sns.displot(data = df, x = "age", bins = 28, col = "positions")

Kernel density estimates

Finally, you don’t have to do histograms. You could also do a Kernel Density Estimate, with kind = "kde" (let’s remove bins = and row =)

sns.displot(data = df, x = "age", kind = "kde")

If you want a separate line for each position, we should indicate that each position needs a different colour/hue with hue = "positions"

sns.displot(data = df, x = "age", hue = "positions", kind = "kde")

Activity 1

Create a histogram which looks at the distribution of heights, with a separate distribution for each position, distinguished by colour. Then, use the multiple = "stack" parameter to make it a bit neater.

Solution

sns.displot(data = df, x = "height_cm", hue = "positions", multiple = "stack")

Relational plots

It seems like players peak in their mid-twenties, but goalkeepers stay for longer. Let’s see if there’s a relationship between players’ age and height

Scatter plots

We’ll start with a scatter plot

sns.relplot(data = df, x = "age", y = "height_cm")

Not much of a trend there, although the bottom-right looks a bit emptier than the rest (could it be that short old players are the first to retire?).

We can use hue = to have a look at positions again

sns.relplot(data = df, x = "age", y = "height_cm", hue = "positions")

Yup, goalkeepers are tall, and everyone else is a jumble.

Line plots

Let’s do a line plot of the average height per age.

sns.relplot(data = df, x = "age", y = "height_cm", kind = "line")

Seems pretty flat, except the ends are a bit weird because there’s not much data. Let’s eliminate everything before 17 and after 38 and plot it

# Create smaller dataframe
condition = (df["age"] > 17) & (df["age"] < 38)
inner_ages = df[condition]

# Line plot
sns.relplot(data = inner_ages, x = "age", y = "height_cm", kind = "line")

Looks a bit shaky but that’s just because it’s zoomed in - notice that we go from 182cm to 184cm. We’ll fix this when we look at matplotlib in the next section.

Combining the two

We can combine our scatter and line plots together.

Make the first plot as normal
For all additional (overlaying) plots, use an axes-level plot instead of sns.relplot() etc. These just draw the points/bars/lines, and are normally behind-the-scenes. There’s one for every plot type, and look like
- sns.lineplot()
- sns.scatterplot()
- sns.boxplot()
- sns.histplot()
- etc.

For example,

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm", hue = "positions")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm")

You can’t include kind = inside an axes level plot

Let’s swap the colour variable from the scatter plot to the line plot

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

Finally, let’s make the scatter dots smaller with s = 10 and grey with color = "grey".

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

Activity 2

It’s really important to become familiar with the documentation. Go to the sns.relplot documentation, and look up the following parameters:

col
col_wrap
col_order
hue_order
legend

Using those parameters, create a scatter plot for age vs height (like above), which meets the following conditions:

Separate the plots for each position.
Apply a different colour to each position.
Arrange the plots in two columns
Remove the legend
Manually order the colours:
1. Attack
2. Midfield
3. Defender
4. Goalkeeper

Hint

For the ordering, you might want to make a list with the desired order, e.g. order = ["Attack", "Midfield", ... ]

Solution

order = ["Attack", "Midfield", "Defender", "Goalkeeper"]

sns.relplot(data = df, x = "age", y = "height_cm", hue = "positions", 
            col = "positions", col_wrap = 2, hue_order = order, 
            col_order = order)

Going deeper with `matplotlib`

Seaborn is great for simple and initial visualisations, but when you need to make adjustments it gets tricky. At its core, seaborn is just a simple way of using matplotlib, an extensive and popular plotting package. It was created as a way of doing MATLAB visualisations with Python, so if you’re coming from there, things will feel familiar.

Pros

Customisable. You can tweak almost every parameter of the visualisations
Fast. It can handle large data
Popular. Lots of people use it, and knowing it will help you collaborate

Cons - a bit programmy

Steep-ish learning curve. Creating basic plots can be easy, but its set up with enough complexity that it takes a bit of work to figure out what’s going on.
Cumbersome. You can tweak almost everything, but this means that it can take some effort to tweak anything.

We’re barely going to touch the matplotlib surface, but we’ll look at some essentials.

Saving plots

Before we move to adjusting the plot, let’s just look at how you save it. While you can do this with seaborn, the matplotlib way is also very simple.

As a first step, you should make a new folder. Navigate using your file explorer to the project and create a new folder called “plots”.

Next, save the current plot with plt.savefig("place_location_here"), and we have to do this at the same time that we make the plot. Let’s save our previous overlaying plot:So run all this code at once:

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

plt.savefig("plots/first_saved_plot.png")

Making modifications

Titles

Notice that the \(y\) axis has an ugly label? That’s because seaborn is just drawing from your dataframe.

We can change axis labels with plt.ylabel()

# Plotting functions
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

# Customisation
plt.ylabel("Height (cm)")

Text(4.8166666666666655, 0.5, 'Height (cm)')

and similarly you could change plt.xlabel(...).

Make sure you run the above line at the same time as your plotting function. You can either * Highlight all the code and press F9 * Make a cell with #%% and press ctrl + enter

We can also change the legend title to “positions” with plt.legend()

# Plotting functions
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

# Customisation
plt.ylabel("Height (cm)")
plt.legend(title = "Positions")

And its location with loc = "lower left"

# Plotting functions
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

# Customisation
plt.ylabel("Height (cm)")
plt.legend(title = "Positions")

And give the whole plot a title with plt.title()

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

# Titles
plt.ylabel("Height (cm)")
plt.legend(title = "Positions")
plt.title("Are players' heights and ages related?")

Text(0.5, 1.0, "Are players' heights and ages related?")

Annotations

You might want to annotate your plot with text and arrows. Text is simple with the plt.text() function; we just need to specify its coordinates and the contents.

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

# Titles
plt.ylabel("Height (cm)")
plt.legend(title = "Positions")
plt.title("Are players' heights and ages related?")

# Annotations
plt.text(38.5, 181, "Not enough\ndata for mean")

Text(38.5, 181, 'Not enough\ndata for mean')

The characters \n mean ‘new line’

We could annotate with arrows too. This is more complex, using the plt.annotate() function:

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

# Titles
plt.ylabel("Height (cm)")
plt.legend(title = "Positions")
plt.title("Are players' heights and ages related?")

# Annotations
plt.text(38.5, 181, "Not enough\ndata for mean")
plt.annotate(text = "No short\nolder players", xy = [37,165], xytext = [40,172],
             arrowprops = dict(width = 1, headwidth = 10, headlength = 10, 
                          facecolor = "black"))

Text(40, 172, 'No short\nolder players')

I’ve split this over multiple lines, but its still one function - check the brackets

All together, our plot has become

Axis limits

The last feature we’ll look at is editing axis limits. Let’s try to make more room in the bottom left for the legend with the functions plt.xlim() and plt.ylim()

# Figure level plot
sns.relplot(data = df, x = "age", y = "height_cm", s = 10, color = "grey")

# Axes level plot (drop the kind = )
sns.lineplot(data = inner_ages, x = "age", y = "height_cm", hue = "positions")

# Titles
plt.ylabel("Height (cm)")
plt.legend(title = "Positions", loc = "lower left")
plt.title("Are players' heights and ages related?")

# Annotations
plt.text(38.5, 181, "Not enough\ndata for mean")
plt.annotate("No short\nolder players", [37,165], [40,172], 
             arrowprops = dict(width = 1,headwidth = 10,headlength = 10, 
                               facecolor = "black"))

# Axis limits
plt.xlim([10,45])
plt.ylim([150,210])

I’m not sure that looks any better, but you get the idea!

Conclusion

As we have seen, seaborn and matplotlib are a powerful tools for visualising data efficiently and aesthetically. A range of other plot types and customisation is available, for inspiration have a look at the seaborn gallery and matplotlib gallery. If any of the content here was too challenging, you have other related issues you’d like to discuss or would simply like to learn more, we the technology training team would love to hear from you. You can contact us at training@library.uq.edu.au.

Here’s a summary of what we’ve covered

Topic	Description
Plotting	Using `seaborn`’s `sns.catplot()` (categorical), `sns.relplot()` (relational, i.e. scatter & line) and `sns.displot()` (distributive) functions, we can make plots by specifying various parameters, e.g. `x = ...`, `y = ...`, `hue = ...`, etc.
Introducing variables into visualisations	We don’t just have to use \(x\)- and \(y\)-axes: we can use colour (`hue = ...`), shape (`style = ...`), size (`size = ...`) and facets (`col = ...`, `row = ...`) to introduce more variables to our visualisations.
Overlaying plots	By combining a figure-level plot (e.g. `sns.catplot()`) with multiple axes-level plots (e.g. `sns.boxplot()`), we can overlay multiple graphs onto the same visualisation
Saving figures	We can use `matplotlib`’s function `plt.savefig(...)` to export our plots
Customisations	The functions `plt.xlabel()`, `plt.ylabel()` and `plt.title()` allow you to customise your plot’s axes. The `plt.legend()` function modifies the legend, and `plt.xlim()` and `plt.ylim()` adjust the axis limits.
Annotations	Use the functions `plt.text()` and `plt.annotate()` to draw lines and text on your visualisation.

Below is a summary of all available* plots in seaborn. Most of these have been examined in either the introductory session or this one, however, there are some which we have not yet looked at. The seaborn documentation and tutorials provide desciptions and advice for all available plots.

*As of v0.12.2

Figure- to Axes-level plot

All the plots below are figure-level. To produce the axes-level plot of the same type, simply use

sns.****plot()

where **** is given in kind = "****" for the corresponding figure-level plot. For example,

sns.relplot( ..., kind = "scatter", ... ) # Figure-level scatter plot
sns.scatterplot( ... ) # Axes-level scatter plot

Relational Plots

Plot Name	Code	Notes
Scatter Plot	`sns.relplot( ... , kind = "scatter", ... )`	Requires numerical data
Line Plot	`sns.relplot( ... , kind = "line", ... )`	Requires numerical data

Distributions

Plot Name	Code	Notes
Histogram	`sns.displot( ... , kind = "hist", ... )`	Can be univariate (`x` only) or bivariate (`x` and `y`)
Kernel Density Estimate	`sns.displot( ... , kind = "kde" , ... )`	Can be univariate (`x` only) or bivariate (`x` and `y`)
ECDF*	`sns.displot( ... , kind = "ecdf", ... )`	.
Rug Plot	`sns.displot( ... , rug = True , ... )`	Combine with another `sns.displot`, plots marginal distributions

*Empirical Cumulative Distribution Functions

Categorical Plots

Plot Name	Code	Notes
Strip Plot	`sns.catplot( ... , kind = "strip" , ... )`	Like a scatterplot for categorical data
Swarm Plot	`sns.catplot( ... , kind = "swarm" , ... )`	.
Box Plot	`sns.catplot( ... , kind = "box" , ... )`	One variable is always interpreted categorically
Violin Plot	`sns.catplot( ... , kind = "violin" , ... )`	One variable is always interpreted categorically
Enhanced Box Plot	`sns.catplot( ... , kind = "boxen", ... )`	A box plot with additional quantiles
Point Plot	`sns.catplot( ... , kind = "point" , ... )`	Like a line plot for categorical data
Bar Plot	`sns.catplot( ... , kind = "bar" , ... )`	Aggregates data
Count Plot	`sns.catplot( ... , kind = "count" , ... )`	A bar plot with the total number of observations

Other Plots

Plot Name	Code	Notes
Pair Plot	`sns.pairplot( ... )`	A form of facetting
Joint Plot	`sns.jointplot( ... )`	.
Regressions	`sns.lmplot( ... )`	.
Residual Plot	`sns.residplot( ... )`	The residuals of a linear regression
Heatmap	`sns.heatmap( ... )`	.
Clustermap	`sns.clustermap( ... )`	.

Setting up

Spyder version

Modules and data

Seaborn for simple visualisations

Categorical plots

Bar plots

Detour - line length

Box plots

Distributions

Histograms

Kernel density estimates

Activity 1

Relational plots

Scatter plots

Line plots

Combining the two

Activity 2

Going deeper with matplotlib

Saving plots

Making modifications

Titles

Annotations

Axis limits

Conclusion

Figure- to Axes-level plot

Relational Plots

Distributions

Categorical Plots

Other Plots

Resources

Going deeper with `matplotlib`