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FIX: Support single row/column grids for nearest shading · matplotlib/matplotlib@4153147 · GitHub
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FIX: Support single row/column grids for nearest shading
Explicitly providing a single row and/or single column grid for pcolor(mesh) with nearest shading, silently produces no plot. This happens because nearest shading interprets the X and Y coordinates as cell centers and then calculates, from neighboring centers, new X and Y coordinates for the cell corners. When a coordinate array contains only one center, its corresponding cell size cannot be determined from neighboring centers. This commit adds a fallback that determines the directional cell size for a single center by taking half of its absolute value on either side. For a zero-valued center -/+ 0.5 is used instead. This allows the cells to be plotted. And a warning is emitted to notify the user.
1 parent 2d0352a commit 4153147

2 files changed

Lines changed: 31 additions & 3 deletions

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‎lib/matplotlib/axes/_axes.py‎

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

‎lib/matplotlib/tests/test_axes.py‎

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

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