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Patch 3.1.1 broke imshow() heatmaps: Tiles cut off on y-axis #15179

@svdHero

Description

@svdHero

Bug report

Bug summary

Matplotlib patch 3.1.1 broke imshow() heatmaps. While with version 3.1.0 the heatmap was plotted correctly, in 3.1.1 the bottom most and the top most heatmap tiles are cut off.
This can even be seen in the Matplotlib online documentation itself, see links below.

Code for reproduction

Taken straight from the Matplotlib online documentation:

import numpy as np
import matplotlib
import matplotlib.pyplot as plt
# sphinx_gallery_thumbnail_number = 2

vegetables = ["cucumber", "tomato", "lettuce", "asparagus",
              "potato", "wheat", "barley"]
farmers = ["Farmer Joe", "Upland Bros.", "Smith Gardening",
           "Agrifun", "Organiculture", "BioGoods Ltd.", "Cornylee Corp."]

harvest = np.array([[0.8, 2.4, 2.5, 3.9, 0.0, 4.0, 0.0],
                    [2.4, 0.0, 4.0, 1.0, 2.7, 0.0, 0.0],
                    [1.1, 2.4, 0.8, 4.3, 1.9, 4.4, 0.0],
                    [0.6, 0.0, 0.3, 0.0, 3.1, 0.0, 0.0],
                    [0.7, 1.7, 0.6, 2.6, 2.2, 6.2, 0.0],
                    [1.3, 1.2, 0.0, 0.0, 0.0, 3.2, 5.1],
                    [0.1, 2.0, 0.0, 1.4, 0.0, 1.9, 6.3]])


fig, ax = plt.subplots()
im = ax.imshow(harvest)

# We want to show all ticks...
ax.set_xticks(np.arange(len(farmers)))
ax.set_yticks(np.arange(len(vegetables)))
# ... and label them with the respective list entries
ax.set_xticklabels(farmers)
ax.set_yticklabels(vegetables)

# Rotate the tick labels and set their alignment.
plt.setp(ax.get_xticklabels(), rotation=45, ha="right",
         rotation_mode="anchor")

# Loop over data dimensions and create text annotations.
for i in range(len(vegetables)):
    for j in range(len(farmers)):
        text = ax.text(j, i, harvest[i, j],
                       ha="center", va="center", color="w")

ax.set_title("Harvest of local farmers (in tons/year)")
fig.tight_layout()
plt.show()

Actual outcome

heatmap_3-1-1
Copied from:
https://matplotlib.org/3.1.1/gallery/images_contours_and_fields/image_annotated_heatmap.html#sphx-glr-gallery-images-contours-and-fields-image-annotated-heatmap-py

Expected outcome

heatmap_3-1-0
Copied from:
https://matplotlib.org/3.1.0/gallery/images_contours_and_fields/image_annotated_heatmap.html#sphx-glr-gallery-images-contours-and-fields-image-annotated-heatmap-py

Matplotlib version

  • Operating system: Windows 8.1 and Ubuntu 18.04
  • Matplotlib version: 3.1.1
  • Matplotlib backend: Qt5Agg
  • Python version: 3.7.4 on Windows and 3.6.8 on Ubuntu

I am using conda on Windows and pip on Ubuntu.

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