Update sample for chart bar stacked

htdocs/samples/chart_bar_stacked.html

@@ -24,10 +24,14 @@
           x: {
             type: 'categorized'
           }
+        },
+        grid: {
+          y: {
+            lines: [{value:0}]
+          }
         }
       });
 
-
       setTimeout(function () {
         chart.groups([['data1', 'data2', 'data3']])
       }, 500);