Fix BoundaryNorm cursor data output (matplotlib#22835)

* Fix matplotlib#21915

* Add test for BoundaryNorm cursor data output

* Use slicing for cleaner code

* Use resampled colormaps

* Test against explicit, predefined labels

Author: yumasheta

lib/matplotlib/artist.py

@@ -12,6 +12,7 @@
 
 import matplotlib as mpl
 from . import _api, cbook
+from .colors import BoundaryNorm
 from .cm import ScalarMappable
 from .path import Path
 from .transforms import (Bbox, IdentityTransform, Transform, TransformedBbox,
@@ -1305,10 +1306,20 @@ def format_cursor_data(self, data):
                 return "[]"
             normed = self.norm(data)
             if np.isfinite(normed):
-                # Midpoints of neighboring color intervals.
-                neighbors = self.norm.inverse(
-                    (int(self.norm(data) * n) + np.array([0, 1])) / n)
-                delta = abs(neighbors - data).max()
+                if isinstance(self.norm, BoundaryNorm):
+                    # not an invertible normalization mapping
+                    cur_idx = np.argmin(np.abs(self.norm.boundaries - data))
+                    neigh_idx = max(0, cur_idx - 1)
+                    # use max diff to prevent delta == 0
+                    delta = np.diff(
+                        self.norm.boundaries[neigh_idx:cur_idx + 2]
+                    ).max()
+
+                else:
+                    # Midpoints of neighboring color intervals.
+                    neighbors = self.norm.inverse(
+                        (int(normed * n) + np.array([0, 1])) / n)
+                    delta = abs(neighbors - data).max()
                 g_sig_digits = cbook._g_sig_digits(data, delta)
             else:
                 g_sig_digits = 3  # Consistent with default below.

lib/matplotlib/tests/test_artist.py

@@ -5,6 +5,8 @@
 
 import pytest
 
+from matplotlib import cm
+import matplotlib.colors as mcolors
 import matplotlib.pyplot as plt
 import matplotlib.patches as mpatches
 import matplotlib.lines as mlines
@@ -372,3 +374,164 @@ class MyArtist4(MyArtist3):
         pass
 
     assert MyArtist4.set is MyArtist3.set
+
+
+def test_format_cursor_data_BoundaryNorm():
+    """Test if cursor data is correct when using BoundaryNorm."""
+    X = np.empty((3, 3))
+    X[0, 0] = 0.9
+    X[0, 1] = 0.99
+    X[0, 2] = 0.999
+    X[1, 0] = -1
+    X[1, 1] = 0
+    X[1, 2] = 1
+    X[2, 0] = 0.09
+    X[2, 1] = 0.009
+    X[2, 2] = 0.0009
+
+    # map range -1..1 to 0..256 in 0.1 steps
+    fig, ax = plt.subplots()
+    fig.suptitle("-1..1 to 0..256 in 0.1")
+    norm = mcolors.BoundaryNorm(np.linspace(-1, 1, 20), 256)
+    img = ax.imshow(X, cmap='RdBu_r', norm=norm)
+
+    labels_list = [
+        "[0.9]",
+        "[1.]",
+        "[1.]",
+        "[-1.0]",
+        "[0.0]",
+        "[1.0]",
+        "[0.09]",
+        "[0.009]",
+        "[0.0009]",
+    ]
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.1))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()
+
+    # map range -1..1 to 0..256 in 0.01 steps
+    fig, ax = plt.subplots()
+    fig.suptitle("-1..1 to 0..256 in 0.01")
+    cmap = cm.get_cmap('RdBu_r', 200)
+    norm = mcolors.BoundaryNorm(np.linspace(-1, 1, 200), 200)
+    img = ax.imshow(X, cmap=cmap, norm=norm)
+
+    labels_list = [
+        "[0.90]",
+        "[0.99]",
+        "[1.0]",
+        "[-1.00]",
+        "[0.00]",
+        "[1.00]",
+        "[0.09]",
+        "[0.009]",
+        "[0.0009]",
+    ]
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.01))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()
+
+    # map range -1..1 to 0..256 in 0.01 steps
+    fig, ax = plt.subplots()
+    fig.suptitle("-1..1 to 0..256 in 0.001")
+    cmap = cm.get_cmap('RdBu_r', 2000)
+    norm = mcolors.BoundaryNorm(np.linspace(-1, 1, 2000), 2000)
+    img = ax.imshow(X, cmap=cmap, norm=norm)
+
+    labels_list = [
+        "[0.900]",
+        "[0.990]",
+        "[0.999]",
+        "[-1.000]",
+        "[0.000]",
+        "[1.000]",
+        "[0.090]",
+        "[0.009]",
+        "[0.0009]",
+    ]
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.001))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()
+
+    # different testing data set with
+    # out of bounds values for 0..1 range
+    X = np.empty((7, 1))
+    X[0] = -1.0
+    X[1] = 0.0
+    X[2] = 0.1
+    X[3] = 0.5
+    X[4] = 0.9
+    X[5] = 1.0
+    X[6] = 2.0
+
+    labels_list = [
+        "[-1.0]",
+        "[0.0]",
+        "[0.1]",
+        "[0.5]",
+        "[0.9]",
+        "[1.0]",
+        "[2.0]",
+    ]
+
+    fig, ax = plt.subplots()
+    fig.suptitle("noclip, neither")
+    norm = mcolors.BoundaryNorm(
+        np.linspace(0, 1, 4, endpoint=True), 256, clip=False, extend='neither')
+    img = ax.imshow(X, cmap='RdBu_r', norm=norm)
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.33))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()
+
+    fig, ax = plt.subplots()
+    fig.suptitle("noclip, min")
+    norm = mcolors.BoundaryNorm(
+        np.linspace(0, 1, 4, endpoint=True), 256, clip=False, extend='min')
+    img = ax.imshow(X, cmap='RdBu_r', norm=norm)
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.33))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()
+
+    fig, ax = plt.subplots()
+    fig.suptitle("noclip, max")
+    norm = mcolors.BoundaryNorm(
+        np.linspace(0, 1, 4, endpoint=True), 256, clip=False, extend='max')
+    img = ax.imshow(X, cmap='RdBu_r', norm=norm)
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.33))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()
+
+    fig, ax = plt.subplots()
+    fig.suptitle("noclip, both")
+    norm = mcolors.BoundaryNorm(
+        np.linspace(0, 1, 4, endpoint=True), 256, clip=False, extend='both')
+    img = ax.imshow(X, cmap='RdBu_r', norm=norm)
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.33))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()
+
+    fig, ax = plt.subplots()
+    fig.suptitle("clip, neither")
+    norm = mcolors.BoundaryNorm(
+        np.linspace(0, 1, 4, endpoint=True), 256, clip=True, extend='neither')
+    img = ax.imshow(X, cmap='RdBu_r', norm=norm)
+    for v, label in zip(X.flat, labels_list):
+        # label = "[{:-#.{}g}]".format(v, cbook._g_sig_digits(v, 0.33))
+        assert img.format_cursor_data(v) == label
+
+    plt.close()