Modify findMinMax

src/axis.js

@@ -589,15 +589,20 @@ c3_chart_internal_fn.findMinMax = function () {
     var minY, maxY, minX, maxX;
     var i,j, id;
 
+    var stacked;
     // detect if we have stacked group
-    var stacked = false;
-    var groups = $$.api.groups();
-    groups.forEach(function(elem){
-        if(elem.length) {
-            stacked = true;
-        }
-    });
-
+    if ($$.ed3Config.stacked) {
+        stacked = true;
+    } else {
+        stacked = false;
+        var groups = $$.api.groups();
+        groups.forEach(function (elem) {
+            if (elem.length) {
+                stacked = true;
+            }
+        });
+    }
+    
     var stackedSum = [];
 
     var allData = $$.api.data();