The one way I know to add a line to a stacked bar chart is to add the range
containing the trend data as a new series to your chart (or create a new
chart and add the trend line as if it were another section of the stacked
bar). Right click on the data series containing the line information, click
on chart type and select the line chart. (If your data doesn't line up
one-to-one you may need to do something more creative with an xy chart type.)
I learned about this stuff on the following web site:
http://peltiertech.com/Excel/Charts/index.html
