Small fix in findMinMax

src/axis.js

@@ -585,13 +585,13 @@ c3_chart_internal_fn.getAxisData =  function(min, max, isAbscissa) {
 };
 
 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;