Fix findMinMax for non-stacked charts

src/axis.js

@@ -592,7 +592,7 @@ c3_chart_internal_fn.findMinMax = function () {
     var stacked;
     // detect if we have stacked group
     if ($$.ed3Config.stacked !== undefined) {
-        stacked = true;
+        stacked = $$.ed3Config.stacked;
     } else {
         stacked = false;
         var groups = $$.api.groups();