Series renderer overide error

[FoxXav]

Hi there !

I started to use Sencha designer 2 since two weeks.
I have looked for a way to add a little bar with another color on the top of each column of a column chart.

The way I found was to override the renderer function of the serie and create a new sprite to add on the graph for each column.

function(sprite, record, attr, index, store) {
    var mark = Ext.create(Ext.draw.Sprite, { /* Here is the new sprite that I add at the top of each column */
      type: 'rect',
      fill: '#FF3333',
      surface: sprite.surface,
      width: attr.width+1,
      x:attr.x,
      y:attr.y,
      height:4
    });
    mark.show(true);
  }

  attr.fill='#4444EE'; /* Here I change the color of the column */
  return attr;
}

If there is a more simple way to do it I'm interested in it however my method work when testing on the browser but in the sencha designer 2 it mess up everything and give me this error :
TypeError: 'undefined' is not an object

See you

sencha designer param.jpg