stacking group only for scatter + modified percy test

Author: dmt0

Diff for: dev/percy/panelTest.json

@@ -73,7 +73,7 @@
         "sizesrc": "x1",
         "sizemode": "diameter"
       },
-      "fill": "tozerox",
+      "stackgroup": 1,
       "mode": "lines+markers+text",
       "line": {
         "dash": "solid",

Diff for: src/default_panels/StyleTracesPanel.js

@@ -355,7 +355,7 @@ const StyleTracesPanel = (props, {localize: _}) => (
       <ContourNumeric label={_('Min Contour')} attr="contours.start" />
       <ContourNumeric label={_('Max Contour')} attr="contours.end" />
     </PlotlySection>
-    <PlotlySection name={_('Stacking')}>
+    <TraceTypeSection name={_('Stacking')} traceTypes={['scatter']} mode="trace">
       <GroupCreator label={_('Group')} prefix={_('Stack')} attr="stackgroup" />
       <Radio
         label={_('Gaps')}
@@ -379,7 +379,7 @@ const StyleTracesPanel = (props, {localize: _}) => (
           {label: _('Percent'), value: 'percent'},
         ]}
       />
-    </PlotlySection>
+    </TraceTypeSection>
     <TraceTypeSection
       name={_('Lines')}
       traceTypes={[