add metatext to layout, xy, polar, ternary & colorbar title.text

Author: etpinard

src/components/titles/index.js

@@ -97,6 +97,10 @@ function draw(gd, titleClass, options) {
         if(!editable) txt = '';
     }
 
+    if(fullLayout.metatext) {
+        txt = Lib.templateString(txt, {metatext: fullLayout.metatext});
+    }
+
     var elShouldExist = txt || editable;
 
     if(!group) {

src/plots/layout_attributes.js

@@ -409,6 +409,17 @@ module.exports = {
         },
         editType: 'modebar'
     },
+    metatext: {
+        valType: 'data_array',
+        editType: 'plot',
+        description: [
+            'Assigns extra meta information that can be used in various `text` attributes.',
+            'Attributes such as the graph, axis and colorbar `title.text` and annotation `text`',
+            'support `metatext`. One can access `metatext` fields using template strings:',
+            '`%{metatext[i]}` where `i` is the index of the `metatext`',
+            'item in question.'
+        ].join(' ')
+    },
     _deprecated: {
         title: {
             valType: 'string',

src/plots/plots.js

@@ -1455,6 +1455,8 @@ plots.supplyLayoutGlobalDefaults = function(layoutIn, layoutOut, formatObj) {
     coerce('modebar.activecolor', Color.addOpacity(modebarDefaultColor, 0.7));
     coerce('modebar.uirevision', uirevision);
 
+    coerce('metatext');
+
     Registry.getComponentMethod(
         'calendars',
         'handleDefaults'