Pivotcharts are evil for a number of reasons - this being one of them.
I generally avoid them and use normal charts off the back of the pivot
tables.  To do this you need to avoid certain doing certain things and
compile your charts in a certain way.  There are various postings about
how to do it but in summary:

- Draw a blank chart by selecting an empty cell away from the pivot
table; clikc the insert chart button and then immediately click finish.
- The most secure way is then to add in the data series manually by
using the 'source data' dialog.
- You can drag and drop the series you want from the pivottable BUT you
must be careful not to select the header row, if you do it will
recognise the pivot table and draw a pivotchart.  So only select the
data and then name the series yourself.
- If you find that the chart suddenly turns in to a pivotchart, undo
what you did and do it a different way - once the chart is built it
won't suddenly change; it's only the initial setup that is the tricky
thing.