added start and end callback for zoom

c3.js

@@ -878,6 +878,8 @@
             zoom_extent: undefined,
             zoom_privileged: false,
             zoom_onzoom: function () {},
+            zoom_onzoomstart: function () {},
+            zoom_onzoomend: function () {},
             interaction_enabled: true,
             onmouseover: function () {},
             onmouseout: function () {},
@@ -4951,6 +4953,7 @@
         $$.zoom = d3.behavior.zoom()
             .on("zoomstart", function () {
                 $$.zoom.altDomain = d3.event.sourceEvent.altKey ? $$.x.orgDomain() : null;
+                config.zoom_onzoomstart.call($$.api, d3.event.sourceEvent);
             })
             .on("zoom", function () {
                 // prevZoomTranslate is needed for the fix of unexpected zoom.translate after remaining zoom
@@ -4960,6 +4963,9 @@
                 $$.redrawForZoom.call($$);
                 prevZoomTranslate = $$.zoom.translate();
                 wheeled = d3.event.sourceEvent.type === 'wheel';
+            })
+            .on('zoomend', function () {
+                config.zoom_onzoomend.call($$.api, $$.x.orgDomain());
             });
         $$.zoom.scale = function (scale) {
             return config.axis_rotated ? this.y(scale) : this.x(scale);

src/config.js

@@ -11,6 +11,8 @@ c3_chart_internal_fn.getDefaultConfig = function () {
         zoom_extent: undefined,
         zoom_privileged: false,
         zoom_onzoom: function () {},
+        zoom_onzoomstart: function () {},
+        zoom_onzoomend: function () {},
         interaction_enabled: true,
         onmouseover: function () {},
         onmouseout: function () {},

src/zoom.js

@@ -4,6 +4,7 @@ c3_chart_internal_fn.initZoom = function () {
     $$.zoom = d3.behavior.zoom()
         .on("zoomstart", function () {
             $$.zoom.altDomain = d3.event.sourceEvent.altKey ? $$.x.orgDomain() : null;
+            config.zoom_onzoomstart.call($$.api, d3.event.sourceEvent);
         })
         .on("zoom", function () {
             // prevZoomTranslate is needed for the fix of unexpected zoom.translate after remaining zoom
@@ -13,6 +14,9 @@ c3_chart_internal_fn.initZoom = function () {
             $$.redrawForZoom.call($$);
             prevZoomTranslate = $$.zoom.translate();
             wheeled = d3.event.sourceEvent.type === 'wheel';
+        })
+        .on('zoomend', function () {
+            config.zoom_onzoomend.call($$.api, $$.x.orgDomain());
         });
     $$.zoom.scale = function (scale) {
         return config.axis_rotated ? this.y(scale) : this.x(scale);