Add sublabel_index and sublabel_skip option for matplotlib backend (

#6375)

Co-authored-by: Simon Høxbro Hansen <simon.hansen@me.com>

holoviews/plotting/mpl/plot.py

@@ -120,12 +120,18 @@ class MPLPlot(DimensionedPlot):
         Allows labeling the subaxes in each plot with various formatters
         including {Alpha}, {alpha}, {numeric} and {roman}.""")
 
+    sublabel_offset = param.Number(default=0, bounds=(0, None), doc="""
+        Allows offsetting the sublabel index.""")
+
     sublabel_position = param.NumericTuple(default=(-0.35, 0.85), doc="""
          Position relative to the plot for placing the optional subfigure label.""")
 
     sublabel_size = param.Number(default=18, doc="""
          Size of optional subfigure label.""")
 
+    sublabel_skip = param.List(default=None, item_type=int, doc="""
+        List of elements to skip when labeling subplots. Numbering starts at 1.""")
+
     projection = param.Parameter(default=None, doc="""
         The projection of the plot axis, default of None is equivalent to
         2D plot, '3d' and 'polar' are also supported by matplotlib by default.
@@ -207,20 +213,29 @@ def _get_fontsize_defaults(self):
 
 
     def _subplot_label(self, axis):
+        if self.sublabel_skip and self.layout_num in self.sublabel_skip:
+            return
         layout_num = self.layout_num if self.subplot else 1
-        if self.sublabel_format and not self.adjoined and layout_num > 0:
+        if self.sublabel_skip:
+            if any(n < 1 for n in self.sublabel_skip):
+                raise ValueError('sublabel_skip values must be greater than 0')
+            sublabel_num = len(set(range(layout_num)) - set(self.sublabel_skip))
+        else:
+            sublabel_num = layout_num
+        sublabel_num += self.sublabel_offset
+        if self.sublabel_format and not self.adjoined and sublabel_num > 0:
             from matplotlib.offsetbox import AnchoredText
             labels = {}
             if '{Alpha}' in self.sublabel_format:
-                labels['Alpha'] = int_to_alpha(layout_num-1)
+                labels['Alpha'] = int_to_alpha(sublabel_num-1)
             elif '{alpha}' in self.sublabel_format:
-                labels['alpha'] = int_to_alpha(layout_num-1, upper=False)
+                labels['alpha'] = int_to_alpha(sublabel_num-1, upper=False)
             elif '{numeric}' in self.sublabel_format:
-                labels['numeric'] = self.layout_num
+                labels['numeric'] = sublabel_num
             elif '{Roman}' in self.sublabel_format:
-                labels['Roman'] = int_to_roman(layout_num)
+                labels['Roman'] = int_to_roman(sublabel_num)
             elif '{roman}' in self.sublabel_format:
-                labels['roman'] = int_to_roman(layout_num).lower()
+                labels['roman'] = int_to_roman(sublabel_num).lower()
             at = AnchoredText(self.sublabel_format.format(**labels), loc=3,
                               bbox_to_anchor=self.sublabel_position, frameon=False,
                               prop=dict(size=self.sublabel_size, weight='bold'),

holoviews/tests/plotting/matplotlib/test_layoutplot.py

@@ -58,3 +58,25 @@ def test_layout_shared_axes_disabled(self):
         cp1, cp2 = plot.traverse(lambda x: x, [CurvePlot])
         self.assertTrue(cp1.handles['axis'].get_ylim(), (1, 3))
         self.assertTrue(cp2.handles['axis'].get_ylim(), (10, 30))
+
+    def test_layout_sublabel_offset(self):
+        from holoviews.plotting.mpl import CurvePlot
+        layout = Curve([]) + Curve([]) + Curve([]) + Curve([])
+        layout.opts(sublabel_offset=1)
+        plot = mpl_renderer.get_plot(layout)
+        cps = plot.traverse(lambda x: x, [CurvePlot])
+        assert cps[0].handles["sublabel"].get_text() == "B"
+        assert cps[1].handles["sublabel"].get_text() == "C"
+        assert cps[2].handles["sublabel"].get_text() == "D"
+        assert cps[3].handles["sublabel"].get_text() == "E"
+
+    def test_layout_sublabel_skip(self):
+        from holoviews.plotting.mpl import CurvePlot
+        layout = Curve([]) + Curve([]) + Curve([]) + Curve([])
+        layout.opts(sublabel_skip=[1, 3])
+        plot = mpl_renderer.get_plot(layout)
+        cps = plot.traverse(lambda x: x, [CurvePlot])
+        assert "sublabel" not in cps[0].handles
+        assert cps[1].handles["sublabel"].get_text() == "A"
+        assert "sublabel" not in cps[2].handles
+        assert cps[3].handles["sublabel"].get_text() == "B"