Add zindex to histogram traces

Author: Farkites

src/traces/histogram/attributes.js

@@ -1,6 +1,7 @@
 'use strict';
 
 var barAttrs = require('../bar/attributes');
+var scatterAttrs = require('../bar/attributes');
 var axisHoverFormat = require('../../plots/cartesian/axis_format_attributes').axisHoverFormat;
 var hovertemplateAttrs = require('../../plots/template_attributes').hovertemplateAttrs;
 var texttemplateAttrs = require('../../plots/template_attributes').texttemplateAttrs;
@@ -248,5 +249,7 @@ module.exports = {
 
     _deprecated: {
         bardir: barAttrs._deprecated.bardir
-    }
+    },
+
+    zindex: scatterAttrs.zindex
 };

src/traces/histogram/defaults.js

@@ -74,4 +74,6 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
     var errorBarsSupplyDefaults = Registry.getComponentMethod('errorbars', 'supplyDefaults');
     errorBarsSupplyDefaults(traceIn, traceOut, lineColor || Color.defaultLine, {axis: 'y'});
     errorBarsSupplyDefaults(traceIn, traceOut, lineColor || Color.defaultLine, {axis: 'x', inherit: 'y'});
+
+    coerce('zindex');
 };