Add missing var.

accountant/frontend/static/js/operations.js

@@ -121,7 +121,7 @@ accountantApp
   };
 
   $scope.brightenColor = function(color) {
-    brightness = 0.2;
+    var brightness = 0.2;
 
     return Highcharts.Color(color).brighten(brightness).get();
   };