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FIX: Support single row/column grids for nearest shading #32405

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FIX: Support single row/column grids for nearest shading #32405

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20 changes: 17 additions & 3 deletions lib/matplotlib/axes/_axes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6560,9 +6560,23 @@ def _interp_grid(X, require_monotonicity=False):
X[:, :-1] + dX,
X[:, [-1]] + dX[:, [-1]]))
else:
# This is just degenerate, but we can't reliably guess
# a dX if there is just one value.
X = np.hstack((X, X))
if require_monotonicity:
# Fallback if coordinate array has only one cell center
# so its corresponding cell size cannot be determined
# from neighboring centers.
_api.warn_external(
"A coordinate array contains a single cell center "
f"along one axis, so {funcname} cannot determine "
"a cell size in that direction to calculate the "
"cell edges. A default cell size is being used; "
f"supply explicit cell edges to {funcname} instead "
"to avoid this warning.")
dX = np.abs(X) * 0.5
dX[dX == 0] = 0.5
hstack = np.ma.hstack if np.ma.isMA(X) else np.hstack
X = hstack((X - dX, X + dX))
else:
X = np.hstack((X, X))
return X

if ncols == Nx:
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14 changes: 14 additions & 0 deletions lib/matplotlib/tests/test_axes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1678,6 +1678,20 @@ def test_pcolorargs():
# bad grid warnings.
ax.pcolormesh(X+10*noise_X, Y+10*noise_Y, Z, shading='auto')
assert len(record) == 2
# If the grid contains a single row and/or column for nearest shading,
# a warning is emitted and a default cell size is used.
x = [2, 4, 6]
y = [5]
Z = np.arange(3).reshape(1, 3)
with pytest.warns(UserWarning,
match='coordinate array contains a single cell center'):
ax.pcolormesh(x, y, Z)
with pytest.warns(UserWarning,
match='coordinate array contains a single cell center'):
ax.pcolormesh(y, x, Z.T)
with pytest.warns(UserWarning,
match='coordinate array contains a single cell center'):
ax.pcolormesh(y, y, Z[:, [0]])


def test_pcolormesh_underflow_error():
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