add barpolar options to subplot panel

Author: dmt0

src/default_panels/GraphSubplotsPanel.js

@@ -60,6 +60,15 @@ const GraphSubplotsPanel = (props, {localize: _}) => (
       <ColorPicker label={_('Plot Background')} attr="bgcolor" />
     </PlotlySection>
 
+    <PlotlySection name={_('Bar Options')}>
+      <Radio
+        label={_('Bar Mode')}
+        attr="barmode"
+        options={[{label: _('Stack'), value: 'stack'}, {label: _('Overlay'), value: 'overlay'}]}
+      />
+      <NumericFraction label={_('Bar Padding')} attr="bargap" showSlider />
+    </PlotlySection>
+
     <PlotlySection name={_('Map Style')}>
       <Dropdown
         label={_('Mapbox Style')}