This is an acknowledged issue with pivot tables and pivot charts. When
they are refreshed, any custom formatting is lost. You can read about it
here:

XL2000: Changing a PivotChart Removes Series Formatting (215904)
  http://support.microsoft.com/?id=215904

Microsoft suggests recording a macro when you reformat the chart by
hand, then using the macro to reformat the chart in the future.

I prefer to make regular charts with pivot table data. You lose a little
interactivity, but you avoid the formatting issue, and you don't
sacrifice lots of other features that pivot charts don't have.

- Jon
-------
Jon Peltier, Microsoft Excel MVP
Peltier Technical Services
Tutorials and Custom Solutions
http://PeltierTech.com/
_______

You could use the Worksheet_Activate or Chart_Activate events to refresh
the pivot tables. For example:

Private Sub Chart_Activate()
  ActiveChart.PivotLayout _
    .PivotTable.RefreshTable
End Sub

Private Sub Worksheet_Activate()
  ActiveSheet.PivotTables(1).RefreshTable
End Sub

To use this code, right-click on the sheet tab, and choose View Code.
Copy and paste the Chart code onto the Chart sheet's module.
Copy and paste the Worksheet code onto the PivotTable sheet's module.