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ENH: allow non-monotonic contour levels #31237

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ENH: allow non-monotonic contour levels #31237

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20aef48
ENH: allow non-monotonic contour
jklymak Mar 4, 2026
ff05e35
TST: test unsorted contour levels
jklymak Mar 4, 2026
c3b5b58
DOC: clarify color, line mapping
jklymak Mar 6, 2026
7e2e5b5
TST: test reversed, but not random levels
jklymak Mar 17, 2026
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18 changes: 11 additions & 7 deletions lib/matplotlib/contour.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1029,8 +1029,12 @@ def _process_contour_level_args(self, args, z_dtype):

if self.filled and len(self.levels) < 2:
raise ValueError("Filled contours require at least 2 levels.")
if len(self.levels) > 1 and np.min(np.diff(self.levels)) <= 0.0:
raise ValueError("Contour levels must be increasing")

# check if filled contours are monotonically increasing:
if (self.filled and len(self.levels) > 1 and
np.any(np.diff(self.levels) <= 0)):
raise ValueError(
"Filled contour levels must be monotonically increasing")

def _process_levels(self):
"""
Expand Down Expand Up @@ -1496,7 +1500,7 @@ def _initialize_x_y(self, z):
*n*=7 is the default.

If array-like, draw contour lines at the specified levels.
The values must be in increasing order.
For filled contours, the values must be in increasing order.

If not specified, a reasonable default is automatically chosen. For
linear scales, this corresponds to *levels=7*. For logarithmic
Expand All @@ -1519,8 +1523,8 @@ def _initialize_x_y(self, z):
The colors of the levels, i.e. the lines for `.contour` and the
areas for `.contourf`.

The sequence is cycled for the levels in ascending order. If the
sequence is shorter than the number of levels, it's repeated.
The colors are mapped to the levels in the order specified, repeating as
necessary if there are fewer colors than levels.

As a shortcut, a single color may be used in place of one-element lists, i.e.
``'red'`` instead of ``['red']`` to color all levels with the same color.
Expand Down Expand Up @@ -1641,8 +1645,8 @@ def _initialize_x_y(self, z):

If a number, all levels will be plotted with this linewidth.

If a sequence, the levels in ascending order will be plotted with
the linewidths in the order specified.
If a sequence, the linewidths are mapped to the levels in order,
repeating as necessary if there are fewer linewidths than levels.

If None, this falls back to :rc:`lines.linewidth`.

Expand Down
16 changes: 16 additions & 0 deletions lib/matplotlib/tests/test_contour.py
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Expand Up @@ -335,6 +335,22 @@ def test_contourf_decreasing_levels():
plt.contourf(z, [1.0, 0.0])


@check_figures_equal()
def test_contour_decreasing_levels(fig_ref, fig_test):
# Check that contour works with decreasing levels:
Z = np.arange(100).reshape(10, 10)
ax_ref = fig_ref.subplots()
ax_test = fig_test.subplots()

levels0 = np.array([6, 10, 20, 30, 33, 53, 80])
colors0 = ['red', 'green', 'blue', 'cyan', 'magenta', 'yellow', 'black']

# sorted:
ax_ref.contour(Z, levels=levels0, colors=np.array(colors0))
# reversed:
ax_test.contour(Z, levels=levels0[::-1], colors=colors0[::-1])


def test_contourf_symmetric_locator():
# github issue 7271
z = np.arange(12).reshape((3, 4))
Expand Down
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