Legend double click hide all but the clicked

src/legend.js

@@ -239,6 +239,10 @@ c3_chart_internal_fn.updateLegend = function (targetIds, options, transitions) {
                 }
             }
         })
+	.on('dblclick', function(id){
+		$$.api.hide();
+		$$.api.show(id);
+	})
         .on('mouseover', function (id) {
             if (config.legend_item_onmouseover) {
                 config.legend_item_onmouseover.call($$, id);