Can you sort by the sales volume?  Are you using a query as the data source
for the chart?  I can't sort out from your description just what is going
on.  It sounds as if a number (dollars or whatever the numbers represent) is
on one axis, and there is one stacked bar per month, so I expect month is
the other axis.  If so, you may need a separate chart for each month, as I
doubt you can rearrange the ordering of the results within a single data
source (chart source data).   I haven't worked with pivot charts very much,
and there does not seem to be much documentation beyond creating charts for
the Northwinds database, so I freely admit I'm speculating a bit here.