fix tooltip test expecting for stacked data

src/test/specs/graph-tooltip-specs.js

@@ -15,6 +15,7 @@ define([
           shared: true
           shared: true
         },
         },
         y_formats: ['ms', 'none'],
         y_formats: ['ms', 'none'],
+	stack: true
       }
       }
     };
     };