Modifying pivot table changes chart type
 

I am having a problem with Excel97. I have a pivot table on a worksheet and
a chart on another sheet. The chart's type is custom (Line - Column on 2
Axes). If I modify the pivot table by adding a pivot field the chart
changes it's type to Column.

Is there any way that I could change the type back to the custom type? I
can't find an event to add the code.