You can stack as many charts as you wish but you must first set the
background of whole chart and the chart area to transparent. do this
by doubble clickin on the outter edges of the chard first and set
transparancy to 0. Then doubble click in the chart area itsel. You
must make sure that you don't get any axis, bars or values, you want
just the background. When the format options dialog box comes up you
set this one to 0 also.
If you are doing multiple overlays it may be helpful to select a color
for each different one. If you select red and set the transparancy to
10% you will get a very light pink tint to this new chart. It helpful
to keeping multiple overlays seperate.
I have also discovered that is is helpful to produce macros to move
specific overlay chart around.
Of course you can always draw lines on top of an existing chart.
I am working in Office 2004 for the Mac. It's contains many advanced
features that your version may not contain including setting
transparancy.
Hope that this helps,
Michael