Right-click on the series you want to change, and select 'Format Data
Series'
On the Axis tab, select Secondary Axis, click.
However, loss of formatting is a known problem with pivot charts, so you
may want to record a macro as you change the axis. There's information
in the following MSKB article:
When I try this, the choice for changing to a secondary axis is greyed out
and not selectable. I know how to make a secondary axis for a regular chart
but it seems as if it can not be applied to a pivot chart?