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pcolormesh grids and shading#
pcolor have a few options for
how grids are laid out and the shading between the grid points.
Generally, if Z has shape (M, N) then the grid X and Y can be
specified with either shape (M+1, N+1) or (M, N), depending on the
argument for the
shading keyword argument. Note that below we specify
vectors x as either length N or N+1 and y as length M or M+1, and
pcolormesh internally makes the mesh matrices X and Y from
the input vectors.
import matplotlib.pyplot as plt import numpy as np
The grid specification with the least assumptions is
and if the grid is one larger than the data in each dimension, i.e. has shape
(M+1, N+1). In that case X and Y specify the corners of quadrilaterals
that are colored with the values in Z. Here we specify the edges of the
(3, 5) quadrilaterals with X and Y that are (4, 6).
nrows = 3 ncols = 5 Z = np.arange(nrows * ncols).reshape(nrows, ncols) x = np.arange(ncols + 1) y = np.arange(nrows + 1) fig, ax = plt.subplots() ax.pcolormesh(x, y, Z, shading='flat', vmin=Z.min(), vmax=Z.max()) def _annotate(ax, x, y, title): # this all gets repeated below: X, Y = np.meshgrid(x, y) ax.plot(X.flat, Y.flat, 'o', color='m') ax.set_xlim(-0.7, 5.2) ax.set_ylim(-0.7, 3.2) ax.set_title(title) _annotate(ax, x, y, "shading='flat'")
Flat Shading, same shape grid#
Often, however, data is provided where X and Y match the shape of Z.
While this makes sense for other
shading types, it is not permitted
shading='flat'. Historically, Matplotlib silently dropped the last
row and column of Z in this case, to match Matlab's behavior. If this
behavior is still desired, simply drop the last row and column manually:
Nearest Shading, same shape grid#
Usually, dropping a row and column of data is not what the user means when
they make X, Y and Z all the same shape. For this case, Matplotlib
shading='nearest' and centers the colored quadrilaterals on the
If a grid that is not the correct shape is passed with
an error is raised.
It's possible that the user would like the code to automatically choose which
to use, in this case
shading='auto' will decide whether to use 'flat' or
'nearest' shading based on the shapes of X, Y and Z.
fig, axs = plt.subplots(2, 1, layout='constrained') ax = axs x = np.arange(ncols) y = np.arange(nrows) ax.pcolormesh(x, y, Z, shading='auto', vmin=Z.min(), vmax=Z.max()) _annotate(ax, x, y, "shading='auto'; X, Y, Z: same shape (nearest)") ax = axs x = np.arange(ncols + 1) y = np.arange(nrows + 1) ax.pcolormesh(x, y, Z, shading='auto', vmin=Z.min(), vmax=Z.max()) _annotate(ax, x, y, "shading='auto'; X, Y one larger than Z (flat)")
Gouraud shading can also be specified, where the color in the quadrilaterals is linearly interpolated between the grid points. The shapes of X, Y, Z must be the same.
The use of the following functions, methods, classes and modules is shown in this example:
Total running time of the script: ( 0 minutes 3.498 seconds)