Update c3.js and c3.min.js

c3.js

@@ -5039,13 +5039,13 @@
     };
 
     c3_chart_internal_fn.findMinMax = function () {
-        var $$ = this, data = $$.data.targets;
+        var $$ = this;
         var minY, maxY, minX, maxX;
         var i,j, id;
 
         var stacked;
         // detect if we have stacked group
-        if ($$.ed3Config.stacked) {
+        if ($$.ed3Config.stacked !== undefined) {
             stacked = true;
         } else {
             stacked = false;