Add "z" variable to hovertemplate in image defined by source attr

Author: antoinerg

src/traces/image/event_data.js

@@ -15,5 +15,6 @@ module.exports = function eventData(out, pt) {
     if(pt.ya) out.yaxis = pt.ya;
     out.color = pt.color;
     out.colormodel = pt.trace.colormodel;
+    if(!out.z) out.z = pt.color;
     return out;
 };

test/jasmine/tests/image_test.js

@@ -686,6 +686,8 @@ describe('image hover:', function() {
           ['y', '125'],
           ['color', '[78, 77, 83, 1]'],
           ['color[0]', '78'],
+          ['z', '[78, 77, 83, 255]'],
+          ['z[0]', '78']
         ].forEach(function(test) {
             it('should support hovertemplate variable ' + test[0], function(done) {
                 var mockCopy = Lib.extendDeep({}, mock);