tick frequency when using seaborn/matplotlib boxplot

Question

I am plotting with seaborn a series of boxplots with

sns.boxplot(full_array)

where full_array contains 200 arrays. Therefore, I have 200 boxplots and ticks on the x-axis from 0 to 200.

The xticks are too close to each other and I would like to show only some of them, for instance, a labeled xtick every 20, or so.

I tried several solutions as those mentioned here but they did not work.

Every time I sample the xticks, I get wrong labels for the ticks, as they get numbered from 0 to N, with unit spacing.

For instance, with the line ax.xaxis.set_major_locator(ticker.MultipleLocator(20)) I get a labelled xtick every 20 but the labels are 1, 2, 3, 4 instead of 20, 40, 60, 80...

Thanks to anyone who's so kind to help.

Solution

The seaborn boxplot uses a FixedLocator and a FixedFormatter, i.e.

print ax.xaxis.get_major_locator()
print ax.xaxis.get_major_formatter()

prints

<matplotlib.ticker.FixedLocator object at 0x000000001FE0D668>
<matplotlib.ticker.FixedFormatter object at 0x000000001FD67B00>

It's therefore not sufficient to set the locator to a MultipleLocator since the ticks' values would still be set by the fixed formatter.

Instead you would want to set a ScalarFormatter, which sets the ticklabels to correspond to the numbers at their position.

import matplotlib.pyplot as plt
import matplotlib.ticker as ticker
import seaborn.apionly as sns
import numpy as np

ax = sns.boxplot(data = np.random.rand(20,30))

ax.xaxis.set_major_locator(ticker.MultipleLocator(5))
ax.xaxis.set_major_formatter(ticker.ScalarFormatter())

plt.show()