stretch histogram support multi-select (#2582)

* stretch histogram support multi-select (but hidden for multiple layers)
* fix updating colorbar when switching between multi/single select

CHANGES.rst

@@ -23,7 +23,7 @@ New Features
 
 - Plugin APIs now include a ``close_in_tray()`` method. [#2562]
 
-- Convert the layer select dropdown in Plot Options into a horizontal panel of buttons. [#2566, #2574]
+- Convert the layer select dropdown in Plot Options into a horizontal panel of buttons. [#2566, #2574, #2582]
 
 Cubeviz
 ^^^^^^^

jdaviz/configs/default/plugins/plot_options/plot_options.py

@@ -705,8 +705,7 @@ def _update_stretch_histogram(self, msg={}):
             return
 
         if not self.viewer.selected or not self.layer.selected:  # pragma: no cover
-            # nothing to plot
-            self.stretch_histogram.clear_all_marks()
+            # nothing to plot, will be hidden in UI
             return
 
         if self.layer_multiselect and len(self.layer.selected) > 1:
@@ -819,6 +818,7 @@ def _update_stretch_histogram(self, msg={}):
              'stretch_hist_nbins',
              'stretch_curve_visible',
              'stretch_function_value', 'stretch_vmin_value', 'stretch_vmax_value',
+             'layer_multiselect'
              )
     @skip_if_no_updates_since_last_active()
     def _update_stretch_curve(self, msg=None):
@@ -827,19 +827,19 @@ def _update_stretch_curve(self, msg=None):
             # or the stretch histogram hasn't been initialized:
             return
 
-        if self.viewer_multiselect or self.layer_multiselect:
-            self.stretch_histogram.clear_marks('stretch_curve', 'stretch_knots', 'colorbar')
+        if self.layer_multiselect and len(self.layer.selected) > 1:
+            # currently only support single-layer, if multiple layers are selected, the plot
+            # will be hidden in the UI
             return
 
-        if len(self.layer.selected_obj):
-            layer = self.layer.selected_obj[0]
-        else:
-            # skip further updates if no data are available:
-            return
+        # could be multi or single-viewer and/or multi-layer with a single entry,
+        # either way, we act on the first entry
+        layer = self.layer.selected_obj[0]
+        while isinstance(layer, list):
+            layer = layer[0]
 
         if isinstance(layer.layer, GroupedSubset):
-            # don't update histogram for subsets:
-            self.stretch_histogram.clear_marks('stretch_curve', 'stretch_knots', 'colorbar')
+            # don't update histogram for subsets, will be hidden in UI
             return
 
         # create the new/updated stretch curve following the colormapping
@@ -945,5 +945,12 @@ def _viewer_is_image_viewer(self):
         from jdaviz.configs.cubeviz.plugins.viewers import CubevizImageView
         from jdaviz.configs.mosviz.plugins.viewers import MosvizImageView, MosvizProfile2DView
 
-        return isinstance(self.viewer.selected_obj, (ImvizImageView, CubevizImageView,
-                                                     MosvizImageView, MosvizProfile2DView))
+        def _is_image_viewer(viewer):
+            return isinstance(viewer, (ImvizImageView, CubevizImageView,
+                                       MosvizImageView, MosvizProfile2DView))
+
+        viewers = self.viewer.selected_obj
+        if not isinstance(viewers, list):
+            viewers = [viewers]
+
+        return np.all([_is_image_viewer(viewer) for viewer in viewers])