Fix BoundaryNorm for multiple colors and one region

dstansby · dstansby · commit 590f4019c158 · 2020-07-03T16:46:26.000+01:00
diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
@@ -1429,9 +1429,9 @@ def __init__(self, boundaries, ncolors, clip=False, *, extend='neither'):
         Parameters
         ----------
         boundaries : array-like
-            Monotonically increasing sequence of boundaries
+            Monotonically increasing sequence of boundaries.
         ncolors : int
-            Number of colors in the colormap to be used
+            Number of colors in the colormap to be used.
         clip : bool, optional
             If clip is ``True``, out of range values are mapped to 0 if they
             are below ``boundaries[0]`` or mapped to ``ncolors - 1`` if they
@@ -1492,6 +1492,7 @@ def __call__(self, value, clip=None):
 
         xx, is_scalar = self.process_value(value)
         mask = np.ma.getmaskarray(xx)
+        # Fill masked values a value above the upper boundary
         xx = np.atleast_1d(xx.filled(self.vmax + 1))
         if clip:
             np.clip(xx, self.vmin, self.vmax, out=xx)
@@ -1500,7 +1501,10 @@ def __call__(self, value, clip=None):
             max_col = self.Ncmap
         iret = np.digitize(xx, self.boundaries) - 1 + self._offset
         if self.Ncmap > self._n_regions:
-            scalefac = (self.Ncmap - 1) / (self._n_regions - 1)
+            if self._n_regions == 1:
+                scalefac = 1
+            else:
+                scalefac = (self.Ncmap - 1) / (self._n_regions - 1)
             iret = (iret * scalefac).astype(np.int16)
         iret[xx < self.vmin] = -1
         iret[xx >= self.vmax] = max_col
diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
@@ -207,6 +207,11 @@ def test_BoundaryNorm():
     bn = mcolors.BoundaryNorm(boundaries, ncolors)
     assert_array_equal(bn(vals), expected)
 
+    # with a single region and interpolation
+    expected = [-1, 0, 0, 0, 3, 3]
+    bn = mcolors.BoundaryNorm([0, 2.2], ncolors)
+    assert_array_equal(bn(vals), expected)
+
     # more boundaries for a third color
     boundaries = [0, 1, 2, 3]
     vals = [-1, 0.1, 1.1, 2.2, 4]
