PivotChart Loss of Format

[ThunderToad]

I have created a PivotChart using the "Line - Column on 2 Axes" custom chart type.
All looks well on initial creation.

However, here lies the problem.
I have two PivotChart Field Buttons as "page fields." When I select/unselect data in these page fields the PivotChart refreshes to reflect accurate data, but the format of my chart changes into a "Column" chart.

Is this a bug/feature/limitation? Or am I missing a key step or setting?

I am using Excel 2000 SP3

Thanks in advance!

TT