Sorting pivot chart field drop down box

[WBS]

I have a complex pivot chart. The x-axis is for dates. I reformatted the sorce data so that it shows calendar years instead of fiscal years along with the months. I now want the x-axis drop down box to show the dates in sequential order like the sorce data, but they are not since I orginally created the pivot chart using a different date structure. The dates are correct just the order is not. How can I reorder the dates in the drop down box without having to recreate the whole pivot chart?

Thanks.