python - Can someone explain this matplotlib pcolormesh quirk?

Question

Plotting an array with pcolormesh with x and y data removes a row of the data

To illustrate what I mean, see the following:

data = np.random.random([5,5])
plt.pcolormesh(data)

results in this 5x5 grid:

But if I want to define the x an y axes for the data like this, pcolormesh creates a 4x4 grid, with the top and right rows missing...

plt.pcolormesh(range(5), range(5), data)

In order to get the full 5x5 plot, I have to do

plt.pcolormesh(range(6), range(6), data)

Is this just a quirk of pcolormesh? Or can someone explain the reasoning why pcolormesh behaves this way?

Answer 1

The behaviour you see is expected. The pcolor documentation states for
pcolor(X, Y, C, **kwargs):

Ideally the dimensions of X and Y should be one greater than those of C; if the dimensions are the same, then the last row and column of C will be ignored.

The same is of course true for pcolormesh.

I'm not sure if this is the place to discuss whether this behaviour is a "quirk", but the underlying idea is that the grid defines the edges of the colored faces. Just as on a number line you have n numbers and n-1 intervals in between.

It actually makes sense to define the edges by the grid, because pcolormesh also allows for unequally spaced grids which would be impossible to define otherwise.