Allow to link to an Overlay

holoviews/core/overlay.py

@@ -287,6 +287,12 @@ def ddims(self):
     def shape(self):
         raise NotImplementedError
 
+    def clone(self, data=None, shared_data=True, new_type=None, link=True,
+              *args, **overrides):
+        if data is None and link:
+            overrides['plot_id'] = self._plot_id
+        return super().clone(data, shared_data=shared_data, new_type=new_type, link=link, *args, **overrides)
+
 
 class NdOverlay(Overlayable, UniformNdMapping, CompositeOverlay):
     """

holoviews/plotting/bokeh/links.py

@@ -85,7 +85,7 @@ def find_links(cls, root_plot):
         Traverses the supplied plot and searches for any Links on
         the plotted objects.
         """
-        plot_fn = lambda x: isinstance(x, GenericElementPlot) and not isinstance(x, GenericOverlayPlot)
+        plot_fn = lambda x: isinstance(x, (GenericElementPlot, GenericOverlayPlot))
         plots = root_plot.traverse(lambda x: x, [plot_fn])
         potentials = [cls.find_link(plot) for plot in plots]
         source_links = [p for p in potentials if p is not None]

holoviews/plotting/bokeh/plot.py

@@ -331,11 +331,11 @@ def init_links(self):
             cb = Link._callbacks['bokeh'][type(link)]
             if src_plot is None or (link._requires_target and tgt_plot is None):
                 continue
+            # The link callback (`cb`) is instantiated (with side-effects).
             callbacks.append(cb(self.root, link, src_plot, tgt_plot))
         return callbacks
 
 
-
 class CompositePlot(BokehPlot):
     """
     CompositePlot is an abstract baseclass for plot types that draw

holoviews/plotting/plot.py

@@ -994,7 +994,7 @@ def link_sources(self):
             zorders = [self.zorder]
 
         if isinstance(self, GenericOverlayPlot) and not self.batched:
-            sources = []
+            sources = [self.hmap.last]
         elif not self.static or isinstance(self.hmap, DynamicMap):
             sources = [o for i, inputs in self.stream_sources.items()
                        for o in inputs if i in zorders]

holoviews/tests/plotting/bokeh/test_links.py

@@ -2,7 +2,7 @@
 import pytest
 
 from holoviews.core.spaces import DynamicMap
-from holoviews.element import Curve, Polygons, Table, Scatter, Path, Points
+from holoviews.element import Curve, Image, Polygons, Table, Scatter, Path, Points
 from holoviews.plotting.links import (Link, RangeToolLink, DataLink)
 
 from bokeh.models import ColumnDataSource
@@ -26,6 +26,34 @@ def test_range_tool_link_callback_single_axis(self):
         self.assertEqual(range_tool.x_range, tgt_plot.handles['x_range'])
         self.assertIs(range_tool.y_range, None)
 
+    def test_range_tool_link_callback_single_axis_overlay_target(self):
+        from bokeh.models import RangeTool
+        array = np.random.rand(100, 2)
+        src = Curve(array)
+        target = Scatter(array, label='a') * Scatter(array, label='b')
+        RangeToolLink(src, target)
+        layout = target + src
+        plot = bokeh_renderer.get_plot(layout)
+        tgt_plot = plot.subplots[(0, 0)].subplots['main']
+        src_plot = plot.subplots[(0, 1)].subplots['main']
+        range_tool = src_plot.state.select_one({'type': RangeTool})
+        self.assertEqual(range_tool.x_range, tgt_plot.handles['x_range'])
+        self.assertIs(range_tool.y_range, None)
+
+    def test_range_tool_link_callback_single_axis_overlay_target_image_source(self):
+        from bokeh.models import RangeTool
+        data = np.random.rand(50, 50)
+        target = Curve(data) * Curve(data)
+        source = Image(np.random.rand(50, 50), bounds=(0, 0, 1, 1))
+        RangeToolLink(source, target)
+        layout = target + source
+        plot = bokeh_renderer.get_plot(layout)
+        tgt_plot = plot.subplots[(0, 0)].subplots['main']
+        src_plot = plot.subplots[(0, 1)].subplots['main']
+        range_tool = src_plot.state.select_one({'type': RangeTool})
+        self.assertEqual(range_tool.x_range, tgt_plot.handles['x_range'])
+        self.assertIs(range_tool.y_range, None)
+
     def test_range_tool_link_callback_both_axes(self):
         from bokeh.models import RangeTool
         array = np.random.rand(100, 2)