add hovertemplate support in histogram

Author: antoinerg

src/traces/histogram/attributes.js

@@ -185,6 +185,8 @@ module.exports = {
         ].join(' ')
     },
 
+    hovertemplate: barAttrs.hovertemplate,
+
     marker: barAttrs.marker,
 
     selected: barAttrs.selected,

src/traces/histogram/defaults.js

@@ -56,6 +56,8 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
     // autobin(x|y) are only included here to appease Plotly.validate
     coerce('autobin' + sampleLetter);
 
+    coerce('hovertemplate');
+
     handleStyleDefaults(traceIn, traceOut, coerce, defaultColor, layout);
 
     // override defaultColor for error bars with defaultLine

src/traces/histogram/hover.js

@@ -27,5 +27,7 @@ module.exports = function hoverPoints(pointData, xval, yval, hovermode) {
         pointData[posLetter + 'Label'] = hoverLabelText(pointData[posLetter + 'a'], di.ph0, di.ph1);
     }
 
+    if(trace.hovermplate) pointData.hovertemplate = trace.hovertemplate;
+
     return pts;
 };