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Jan 14, 2019
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Improve docs on contourf extend #13047

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44 changes: 36 additions & 8 deletions lib/matplotlib/contour.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1766,14 +1766,42 @@ def _initialize_x_y(self, z):
are not given explicitly via *levels*.
Defaults to `~.ticker.MaxNLocator`.

extend : {'neither', 'both', 'min', 'max'}, optional
Unless this is 'neither', contour levels are automatically
added to one or both ends of the range so that all data
are included. These added ranges are then mapped to the
special colormap values which default to the ends of the
colormap range, but can be set via
:meth:`matplotlib.colors.Colormap.set_under` and
:meth:`matplotlib.colors.Colormap.set_over` methods.
extend : {'neither', 'both', 'min', 'max'}, optional, default: \
'neither'
Determines the ``contourf``-coloring of values that are outside the
*levels* range.

If 'neither', values outside the *levels* range are not colored.
If 'min', 'max' or 'both', color the values below, above or below
and above the *levels* range.

Values below ``min(levels)`` and above ``max(levels)`` are mapped
to the under/over values of the `.Colormap`. Note, that most
colormaps do not have dedicated colors for these by default, so
that the over and under values are the edge values of the colormap.
You may want to set these values explicitly using
`.Colormap.set_under` and `.Colormap.set_over`.

.. note::

An exising `.QuadContourSet` does not get notified if
properties of its colormap are changed. Therefore, an explicit
call `.QuadContourSet.changed()` is needed after modifying the
colormap. The explicit call can be left out, if a colorbar is
assigned to the `.QuadContourSet` because it interally calls
`.QuadContourSet.changed()`.

Example::

x = np.arange(1, 10)
y = x.reshape(-1, 1)
h = x * y

cs = plt.contourf(h, levels=[10, 30, 50],
colors=['#808080', '#A0A0A0', '#C0C0C0'], extend='both')
cs.cmap.set_over('red')
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It is probably better to show updating the color map before creating the contour and then passing that in to contourf. I think that is a bit for idiomatic and does not expose this updating edge-case in the examples.

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Ah, the point is we are creating the colormap on-the-fly so can not do that.

We have not noticed the lack of update on images because we unconditionally consult the colormap every time we draw.

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@timhoffm timhoffm Dec 25, 2018 •
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Updating a QuadContourSet on each draw (to get automatic updates and the same behavior as with images) would essentially mean that we have to run the full code of changed(). Not quite sure if we want to do this.

I'm just being defensive here and clearly document the current behavior.

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I agree this is the conservative path here.

cs.cmap.set_under('blue')
cs.changed()

xunits, yunits : registered units, optional
Override axis units by specifying an instance of a
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