This is a known problem with pivot charts. There's information in the
following MSKB article, which suggests using a macro to reapply the
formatting:

Changing a PivotChart removes series formatting in Excel
http://support.microsoft.com/?id=215904

hannah220507 wrote:
I created a whole heap of Pivot table charts. I also selected a default chart
type to use whereby Excel automatically used my default chart type every time
I created a new chart for pivot tables. Unfortunately when I press the
refresh button for my pivot table reports Excel automatically changes all of
my charts back to the original chart type and ignores my default chart type
which I selected.

Does anyone know how to fix this?


--
Debra Dalgleish
Contextures
http://www.contextures.com/tiptech.html