Rotating axis labels in matplotlib and seaborn
Rotating axis labels is the classic example of something that seems like an obvious tweak, but can be tricky.
There's a common pattern which often occurs when working with charting libraries: drawing charts with all the defaults seems very straightforward, but when we want to change some aspect of the chart things get complicated. This pattern is even more noticable when working with a high-level library like seaborn - the library does all sorts of clever things to make our life easier, and lets us draw sophisticated, beautiful charts, so it's frustrating when we want to change something that feels like it should be simple.
In this article, we'll take a look at the classic example of this phenomenon - rotating axis tick labels. This seems like such a common thing that it should be easy, but it's one of the most commonly asked questions on StackOverflow for both seaborn and matplotlib. As an example dataset, we'll look at a table of Olympic medal winners. We can load it into pandas directly from a URL:
import pandas as pd
data = pd.read_csv("https://raw.githubusercontent.com/mojones/binders/master/olympics.csv", sep="\t")
data
Each row is a single medal, and we have a bunch of different information like where and when the event took place, the classification of the event, and the name of the athlete that won.
We'll start with something simple; let's grab all the events for the 1980 games and see how many fall into each type of sport:
import seaborn as sns
import matplotlib.pyplot as plt
# set the figure size
plt.figure(figsize=(10,5))
# draw the chart
chart = sns.countplot(
data=data[data['Year'] == 1980],
x='Sport',
palette='Set1'
)
Here we have the classic problem with categorical data: we need to display all the labels and because some of them are quite long, they overlap. How are we going to rotate them? The key is to look at what type of object we've created. What is the type of the return value from the countplot() function, which we have stored in chart?
type(chart)
Looks like chart is a matplotlib AxesSubplot object. This actually doesn't help us very much - if we go searching for the documentation for AxesSubplot we won't find anything useful. Instead, we have to know that an AxesSubplot is a type of Axes object, and now we can go look up the documentation for Axes in which we find the set_xticklabels() method.
Looking at the documentation for set_xticklabels() we don't actually see any obvious reference to rotation. The clue we're looking for is in the "Other parameters" section at the end, where it tells us that we can supply a list of keyword arguments that are properties of Text objects.
Finally, in the documentation for Text objects we can see a list of the properties, including rotation. This was a long journey! but hopefully it will pay off - there are lots of other useful properties here as well. Now we can finally set the rotation:
plt.figure(figsize=(10,5))
chart = sns.countplot(
data=data[data['Year'] == 1980],
x='Sport',
palette='Set1'
)
chart.set_xticklabels(rotation=45)
Disaster! We need to pass set_xticklabels() a list of the actual labels we want to use. Since we don't want to change the labels themselves, we can just call get_xticklabels():
plt.figure(figsize=(10,5))
chart = sns.countplot(
data=data[data['Year'] == 1980],
x='Sport',
palette='Set1'
)
chart.set_xticklabels(chart.get_xticklabels(), rotation=45)
None #don't show the label objects
This looks better, but notice how the "Modern Pentathlon" label is running into the "Sailing" label? That's because the labels have been rotated about their center - which also makes it hard to see which label belongs to which bar. We should also set the horizontal alignment to "right":
plt.figure(figsize=(10,5))
chart = sns.countplot(
data=data[data['Year'] == 1980],
x='Sport',
palette='Set1'
)
chart.set_xticklabels(chart.get_xticklabels(), rotation=45, horizontalalignment='right')
None #don't show the label objects
And just to show a few more things that we can do with set_xticklabels() we'll also set the font weight to be a bit lighter, and the font size to be a bit bigger:
plt.figure(figsize=(10,5))
chart = sns.countplot(
data=data[data['Year'] == 1980],
x='Sport',
palette='Set1'
)
chart.set_xticklabels(
chart.get_xticklabels(),
rotation=45,
horizontalalignment='right',
fontweight='light',
fontsize='x-large'
)
None #don't show the label objects
In all of these examples, we've been using the object-oriented interface to matplotlib - notice that we're calling set_xticklabels() directly on the chart object.
Another object is to use the pyplot interface. There's a method simply called xticks() which we could use like this:
import matplotlib.pyplot as plt
plt.figure(figsize=(10,5))
chart = sns.countplot(
data=data[data['Year'] == 1980],
x='Sport',
palette='Set1'
)
plt.xticks(
rotation=45,
horizontalalignment='right',
fontweight='light',
fontsize='x-large'
)
None #don't show the label objects
Notice that when we do it this way the list of labels is optional, so we don't need to call get_xticklabels().
Althought the pyplot interface is easier to use in this case, in general I find it clearer to use the object-oriented interface, as it tends to be more explicit.
Everything that we've seen above applies if we're using matplotlib directly instead of seaborn: once we have an Axes object, we can call set_xticklabels() on it. Let's do the same thing using pandas's built in plotting function:
chart = data[data['Year'] == 1980]['Sport'].value_counts().plot(kind='bar')
chart.set_xticklabels(chart.get_xticklabels(), rotation=45, horizontalalignment='right')
None
chart = sns.catplot(
data=data[data['Year'].isin([1980, 2008])],
x='Sport',
kind='count',
palette='Set1',
row='Year',
aspect=3,
height=3
)
As before, the labels need to be rotated. Let's try the approach that we used before:
chart = sns.catplot(
data=data[data['Year'].isin([1980, 2008])],
x='Sport',
kind='count',
palette='Set1',
row='Year',
aspect=3,
height=3
)
chart.set_xticklabels(chart.get_xticklabels(), rotation=45, horizontalalignment='right')
We run into an error. Note that the missing attribute is not set_xticklabels() but get_xticklabels(). The reason why this approach worked for countplot() and not for factorplot() is that the output from countplot() is a single Axes object, as we saw above, but the output from factorplot() is a seaborn FacetGrid object:
type(chart)
whose job is to store a collection of multiple axes - two in this case. So how to rotate the labels? It turns out that FacetGrid has its own version of set_xticklabels that will take care of things:
chart = sns.catplot(
data=data[data['Year'].isin([1980, 2008])],
x='Sport',
kind='count',
palette='Set1',
row='Year',
aspect=3,
height=3
)
chart.set_xticklabels(rotation=65, horizontalalignment='right')
None
The pyplot interface that we saw earlier also works fine:
chart = sns.catplot(
data=data[data['Year'].isin([1980, 2008])],
x='Sport',
kind='count',
palette='Set1',
row='Year',
aspect=3,
height=3
)
plt.xticks(rotation=65, horizontalalignment='right')
None
And, of course, everything that we've done here will work for y-axis labels as well - we typically don't need to change their rotation, but we might want to set their other properties. As an example, let's count how many medals were won at each Olypmic games for each country in each year. To keep the dataset managable, we'll just look at countries that have won more than 500 metals in total:
by_sport = (data
.groupby('Country')
.filter(lambda x : len(x) > 500)
.groupby(['Country', 'Year'])
.size()
.unstack()
)
by_sport
If the use of two groupby() method calls is confusing, take a look at this article on grouping. The first one just gives us the rows belonging to countries that have won more than 500 medals; the second one does the aggregation and fills in missing data. The natural way to display a table like this is as a heatmap:
plt.figure(figsize=(10,10))
g = sns.heatmap(
by_sport,
square=True, # make cells square
cbar_kws={'fraction' : 0.01}, # shrink colour bar
cmap='OrRd', # use orange/red colour map
linewidth=1 # space between cells
)
This example is perfectly readable, but by way of an example we'll rotate both the x and y axis labels:
plt.figure(figsize=(10,10))
g = sns.heatmap(
by_sport,
square=True,
cbar_kws={'fraction' : 0.01},
cmap='OrRd',
linewidth=1
)
g.set_xticklabels(g.get_xticklabels(), rotation=45, horizontalalignment='right')
g.set_yticklabels(g.get_yticklabels(), rotation=45, horizontalalignment='right')
None # prevent the list of label objects showing up annoyingly in the output
OK, I think that covers it. That was an agonizingly long article to read just about rotating labels, but hopefully it's given you an insight into what's going on. It all comes down to understanding what type of object you're working with - an Axes, a FacetGrid, or a PairGrid.
If you encounter a situation where none of these work, drop me an email at martin@drawingwithdata.com and I'll update this article!