pcolormesh has improved transparency handling by enabling sub-pixel snapping

Due to how the snapping keyword argument was getting passed to the AGG backend,

previous versions of Matplotlib would appear to show lines between the grid

edges of a mesh with transparency. This version now applies snapping

by default. To restore the old behavior (e.g., for test images), you may set

:rc:`pcolormesh.snap` to `False`.

.. plot::

import matplotlib.pyplot as plt

import numpy as np

# Use old pcolormesh snapping values

plt.rcParams['pcolormesh.snap'] = False

fig, ax = plt.subplots()

xx, yy = np.meshgrid(np.arange(10), np.arange(10))

z = (xx + 1) * (yy + 1)

mesh = ax.pcolormesh(xx, yy, z, shading='auto', alpha=0.5)

fig.colorbar(mesh, orientation='vertical')

ax.set_title('Before (pcolormesh.snap = False)')

Note that there are lines between the grid boundaries of the main plot which

are not the same transparency. The colorbar also shows these lines when a

transparency is added to the colormap because internally it uses pcolormesh

to draw the colorbar. With snapping on by default (below), the lines

at the grid boundaries disappear.

.. plot::

import matplotlib.pyplot as plt

import numpy as np

fig, ax = plt.subplots()

xx, yy = np.meshgrid(np.arange(10), np.arange(10))

z = (xx + 1) * (yy + 1)

mesh = ax.pcolormesh(xx, yy, z, shading='auto', alpha=0.5)

fig.colorbar(mesh, orientation='vertical')

ax.set_title('After (default: pcolormesh.snap = True)')