.. only:: html
.. note::
:class: sphx-glr-download-link-note
Click :ref:`here ` to download the full example code
.. rst-class:: sphx-glr-example-title
.. _sphx_glr_gallery_images_contours_and_fields_pcolormesh_grids.py:
============================
pcolormesh grids and shading
============================
`.axes.Axes.pcolormesh` and `~.axes.Axes.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
`~.axes.Axes.pcolormesh` internally makes the mesh matrices *X* and *Y* from
the input vectors.
.. code-block:: default
import matplotlib
import matplotlib.pyplot as plt
import numpy as np
Flat Shading
------------
The grid specification with the least assumptions is ``shading='flat'``
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)*.
.. code-block:: default
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'")
.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_001.png
:alt: shading='flat'
:class: sphx-glr-single-img
Flat Shading, same shape grid
-----------------------------
Often, however, data is provided where *X* and *Y* match the shape of *Z*.
As of Matplotlib v3.3, ``shading='flat'`` is deprecated when this is the
case, a warning is raised, and the last row and column of *Z* are dropped.
This dropping of the last row and column is what Matplotlib did silently
previous to v3.3, and is compatible with what Matlab does.
.. code-block:: default
x = np.arange(ncols) # note *not* ncols + 1 as before
y = np.arange(nrows)
fig, ax = plt.subplots()
ax.pcolormesh(x, y, Z, shading='flat', vmin=Z.min(), vmax=Z.max())
_annotate(ax, x, y, "shading='flat': X, Y, C same shape")
.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_002.png
:alt: shading='flat': X, Y, C same shape
:class: sphx-glr-single-img
.. rst-class:: sphx-glr-script-out
Out:
.. code-block:: none
/root/matplotlib/examples/images_contours_and_fields/pcolormesh_grids.py:66: MatplotlibDeprecationWarning: shading='flat' when X and Y have the same dimensions as C is deprecated since 3.3. Either specify the corners of the quadrilaterals with X and Y, or pass shading='auto', 'nearest' or 'gouraud', or set rcParams['pcolor.shading']. This will become an error two minor releases later.
ax.pcolormesh(x, y, Z, shading='flat', vmin=Z.min(), vmax=Z.max())
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
allows ``shading='nearest'`` and centers the colored quadrilaterals on the
grid points.
If a grid that is not the correct shape is passed with ``shading='nearest'``
an error is raised.
.. code-block:: default
fig, ax = plt.subplots()
ax.pcolormesh(x, y, Z, shading='nearest', vmin=Z.min(), vmax=Z.max())
_annotate(ax, x, y, "shading='nearest'")
.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_003.png
:alt: shading='nearest'
:class: sphx-glr-single-img
Auto Shading
------------
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*.
.. code-block:: default
fig, axs = plt.subplots(2, 1, constrained_layout=True)
ax = axs[0]
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[1]
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)")
.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_004.png
:alt: shading='auto'; X, Y, Z: same shape (nearest), shading='auto'; X, Y one larger than Z (flat)
:class: sphx-glr-single-img
Gouraud Shading
---------------
`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.
.. code-block:: default
fig, ax = plt.subplots(constrained_layout=True)
x = np.arange(ncols)
y = np.arange(nrows)
ax.pcolormesh(x, y, Z, shading='gouraud', vmin=Z.min(), vmax=Z.max())
_annotate(ax, x, y, "shading='gouraud'; X, Y same shape as Z")
plt.show()
.. image:: /gallery/images_contours_and_fields/images/sphx_glr_pcolormesh_grids_005.png
:alt: shading='gouraud'; X, Y same shape as Z
:class: sphx-glr-single-img
------------
References
""""""""""
The use of the following functions and methods is shown in this example:
.. code-block:: default
matplotlib.axes.Axes.pcolormesh
matplotlib.pyplot.pcolormesh
.. rst-class:: sphx-glr-script-out
Out:
.. code-block:: none
.. rst-class:: sphx-glr-timing
**Total running time of the script:** ( 0 minutes 1.855 seconds)
.. _sphx_glr_download_gallery_images_contours_and_fields_pcolormesh_grids.py:
.. only :: html
.. container:: sphx-glr-footer
:class: sphx-glr-footer-example
.. container:: sphx-glr-download sphx-glr-download-python
:download:`Download Python source code: pcolormesh_grids.py `
.. container:: sphx-glr-download sphx-glr-download-jupyter
:download:`Download Jupyter notebook: pcolormesh_grids.ipynb `
.. only:: html
.. rst-class:: sphx-glr-signature
Keywords: matplotlib code example, codex, python plot, pyplot
`Gallery generated by Sphinx-Gallery
`_