FIX: colorbars with NoNorm

Closes matplotlib#21870

This adds another special-case path for NoNorm and tweaks the contents of another.
These special-cases are required because NoNorm (like BoundaryNorm) has a
different return value than ever other Norm (it directly returns integers to
index into the LUT rather than [0, 1] that is later mapped into the LUT.

Co-authored-by: Jody Klymak <[email protected]>

Author: tacaswell

lib/matplotlib/colorbar.py

@@ -839,7 +839,8 @@ def _get_ticker_locator_formatter(self):
                 # default locator:
                 nv = len(self._values)
                 base = 1 + int(nv / 10)
-                locator = ticker.IndexLocator(base=base, offset=0)
+                # put ticks on integers between the boundaries of NoNorm...
+                locator = ticker.IndexLocator(base=base, offset=.5)
 
         if minorlocator is None:
             minorlocator = ticker.NullLocator()
@@ -1097,6 +1098,9 @@ def _process_values(self):
         # otherwise values are set from the boundaries
         if isinstance(self.norm, colors.BoundaryNorm):
             b = self.norm.boundaries
+        elif isinstance(self.norm, colors.NoNorm):
+            # NoNorm has N blocks, so N+1 boundaries, centered on integers:
+            b = np.arange(self.cmap.N + 1) - .5
         else:
             # otherwise make the boundaries from the size of the cmap:
             N = self.cmap.N + 1

lib/matplotlib/tests/baseline_images/test_colorbar/nonorm_colorbars.svg

@@ -7,10 +7,11 @@
 from matplotlib import rc_context
 from matplotlib.testing.decorators import image_comparison
 import matplotlib.pyplot as plt
-from matplotlib.colors import BoundaryNorm, LogNorm, PowerNorm, Normalize
+from matplotlib.colors import (
+    BoundaryNorm, LogNorm, PowerNorm, Normalize, NoNorm
+)
 from matplotlib.colorbar import Colorbar
 from matplotlib.ticker import FixedLocator
-
 from matplotlib.testing.decorators import check_figures_equal
 
 
@@ -913,3 +914,18 @@ def test_negative_boundarynorm():
     cb = fig.colorbar(cm.ScalarMappable(cmap=cmap, norm=norm), cax=ax)
     np.testing.assert_allclose(cb.ax.get_ylim(), [clevs[0], clevs[-1]])
     np.testing.assert_allclose(cb.ax.get_yticks(), clevs)
+
+
+@image_comparison(['nonorm_colorbars.svg'], remove_text=False,
+                  style='mpl20')
+def test_nonorm():
+    plt.rcParams['svg.fonttype'] = 'none'
+    data = [1, 2, 3, 4, 5]
+
+    fig, ax = plt.subplots(figsize=(6, 1))
+    fig.subplots_adjust(bottom=0.5)
+
+    norm = NoNorm(vmin=min(data), vmax=max(data))
+    cmap = cm.get_cmap("viridis", len(data))
+    mappable = cm.ScalarMappable(norm=norm, cmap=cmap)
+    cbar = fig.colorbar(mappable, cax=ax, orientation="horizontal")