Matplotlib: Move ticklabels between ticks

Question

I want to create a visualization of a confusion matrix using matplotlib. Parameters to the methods shown below are the class labels (alphabet), the classification results as

Answer 1

Alternatively: have you tried im = ax.matshow(data, cmap='gray') instead of imshow()? This should also place the ticklabels on the correct position.

Answer 2

As you've noticed, they're centered by default and you're overriding the default behavior by specifying extent=[0, width, height, 0].

There are a number of ways to handle this. One is to use pcolor and set the edgecolors and linestyles to look like the gridlines (you actually need pcolor and not pcolormesh for this to work). However, you'll have to change the extents to get the ticks in the center as imshow does by default.

import matplotlib.pyplot as plt
import numpy as np

data = np.random.random((10,10))
labels = 'abcdefghij'

fig, ax = plt.subplots()
im = ax.pcolor(data, cmap='gray', edgecolor='black', linestyle=':', lw=1)
fig.colorbar(im)

# Shift ticks to be at 0.5, 1.5, etc
for axis in [ax.xaxis, ax.yaxis]:
    axis.set(ticks=np.arange(0.5, len(labels)), ticklabels=labels)

plt.show()

Alternatively, you could turn on the minor grid and place it at the pixel boundaries. Because you want fixed labels, we'll just set everything manually. Otherwise, a MultipleLocator would make more sense:

import matplotlib.pyplot as plt
import numpy as np

data = np.random.random((10,10))
labels = 'abcdefghij'

fig, ax = plt.subplots()
im = ax.imshow(data, cmap='gray', interpolation='none')
fig.colorbar(im)

# Set the major ticks at the centers and minor tick at the edges
locs = np.arange(len(labels))
for axis in [ax.xaxis, ax.yaxis]:
    axis.set_ticks(locs + 0.5, minor=True)
    axis.set(ticks=locs, ticklabels=labels)

# Turn on the grid for the minor ticks
ax.grid(True, which='minor')

plt.show()