Why Are My Seaborn Heatmaps Cut Off at the First and Last Rows?

Heatmap Cut in Half: First and Last Row Disappearing

When generating heatmaps in Python using seaborn or correlation matrices using matplotlib, users commonly encounter an issue where the first and last rows are cut in half. This peculiar behavior has been observed even in simple code examples.

For instance, consider the following code:

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

data = pd.read_csv('https://raw.githubusercontent.com/resbaz/r-novice-gapminder-files/master/data/gapminder-FiveYearData.csv')
plt.figure(figsize=(10,5))
sns.heatmap(data.corr())
plt.show()

Running this code should produce a heatmap, but instead, it displays an incomplete one with the labels at the y-axis in the correct position, but half of the first and last rows missing.

The root cause of this issue lies with matplotlib 3.1.1, which introduced a bug that breaks seaborn heatmaps and inverted axes with fixed ticks. To resolve this issue, users can take the following steps:

• Revert to matplotlib 3.1.0: Downgrading to the previous version of matplotlib may restore the heatmap functionality.
• Use matplotlib 3.1.2 or higher: If possible, updating to a version of matplotlib that includes the fix can resolve the problem.
• Set heatmap limits manually: As a workaround, explicitly setting the limits of the heatmap using the ax.set_ylim function can help to restore the correct display.

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