Offer a diverging colormap

de_shell/plotting/colormaps.py

@@ -21,7 +21,11 @@
     "magma": "magma",
     "cividis": "cividis",
     "turbo": "turbo",
-    "CET_R1": "CET_R1",   # diverging
+    "CET_R1": "CET_R1",   # a rainbow, not a diverging map: its middle is yellow
+    "CET_D1": "CET_D1",   # diverging blue-white-red, matplotlib's coolwarm
 }
 
 DEFAULT_COLORMAP = "gray"
+#: For a signed map (a strain, a field component): white at the middle, so a
+#: range centered on zero shows zero as white.
+DIVERGING_COLORMAP = "CET_D1"

tests/test_figure.py

@@ -237,3 +237,27 @@ def test_hiding_the_pill_on_a_closed_figure_is_a_no_op(self):
     def test_every_figure_frame_relays_its_readout_to_the_host(self):
         head = fill_iframe_html("<head></head><body></body>").split("<body>")[0]
         assert "apl:readout" in head and "apl_readout" in head
+
+
+class TestColormaps:
+    def test_every_name_is_its_own_table(self):
+        # A misspelled name falls back to the grey ramp without an error, and
+        # some names are aliases of others; either way the picker would offer
+        # two entries that draw the same picture.
+        from anyplotlib._utils import _build_colormap_lut
+
+        from de_shell.plotting.colormaps import COLORMAPS
+
+        tables = {name: bytes(np.asarray(_build_colormap_lut(name), np.uint8)) for name in COLORMAPS}
+        assert len(set(tables.values())) == len(tables)
+
+    def test_the_diverging_map_is_white_in_the_middle(self):
+        from anyplotlib._utils import _build_colormap_lut
+
+        from de_shell.plotting.colormaps import COLORMAPS, DIVERGING_COLORMAP
+
+        assert DIVERGING_COLORMAP in COLORMAPS
+        table = np.asarray(_build_colormap_lut(DIVERGING_COLORMAP), float)
+        low, middle, high = table[0], table[len(table) // 2], table[-1]
+        assert middle.min() > 220, "zero should read as white"
+        assert low[2] > low[0] and high[0] > high[2], "blue below zero, red above"

upcoming_changes/20.new_feature.rst

@@ -0,0 +1 @@
+The colormap picker offers ``CET_D1``, a diverging blue-white-red map (matplotlib's ``coolwarm``), and ``DIVERGING_COLORMAP`` names it for signed images. ``CET_R1``, which was listed as the diverging map, is a rainbow.