add 'modeBarButtonsToAdd' config arg:

- takes an array of buttons config objects (as the
  defined for the default buttons in modebar/buttons.js)
- these 'buttons to add' get appended an additional group
  of buttons passed to the ModeBar constructor

Author: etpinard

src/components/modebar/manage.js

@@ -40,16 +40,26 @@ module.exports = function manageModeBar(gd) {
         ].join(' '));
     }
 
-//     if(!Array.isArray(context.m))
+    if(!Array.isArray(context.modeBarButtonsToAdd)) {
+        throw new Error([
+            '*modeBarButtonsToAdd* configuration options',
+            'must be an array.'
+        ].join(' '));
+    }
+
 
-    var buttonGroups = getButtonGroups(fullLayout, context.modeBarButtonsToRemove);
+        buttonGroups = getButtonGroups(
+            fullLayout,
+            context.modeBarButtonsToRemove,
+            context.modeBarButtonsToAdd
+        );
 
     if(modeBar) modeBar.update(gd, buttonGroups);
     else fullLayout._modeBar = createModeBar(gd, buttonGroups);
 };
 
 // logic behind which buttons are displayed by default
-function getButtonGroups(fullLayout, buttonsToRemove) {
+function getButtonGroups(fullLayout, buttonsToRemove, buttonsToAdd) {
     var groups = [];
 
     function addGroup(newGroup) {
@@ -97,6 +107,8 @@ function getButtonGroups(fullLayout, buttonsToRemove) {
         addGroup(['hoverClosestPie']);
     }
 
+    if(buttonsToAdd.length) groups.push(buttonsToAdd);
+
     return groups;
 }
 

src/plot_api/plot_config.js

@@ -58,10 +58,14 @@ module.exports = {
     // display the mode bar (true, false, or 'hover')
     displayModeBar: 'hover',
 
-    // remove modebar button by name
+    // remove mode bar button by name
     // (see ./components/modebar/buttons.js for the list of names)
     modeBarButtonsToRemove: [],
 
+    // add mode bar button using config objects
+    // (see ./components/modebar/buttons.js for list of arguments)
+    modeBarButtonsToAdd: [],
+
     // add the plotly logo on the end of the mode bar
     displaylogo: true,