If you're using actual dates, it's pretty easy. You just need a simple
modification of the data so you get two series (one for each color). Here is
12 months of data, arranged to plot before May 1 in red and after May 1 in
blue:
Red Blue
1/1/2008 11 0
2/1/2008 7 0
3/1/2008 12 0
4/1/2008 7 0
5/1/2008 15 0
5/1/2008 0 15
6/1/2008 0 13
7/1/2008 0 16
8/1/2008 0 12
9/1/2008 0 17
10/1/2008 0 16
11/1/2008 0 18
12/1/2008 0 19
1/1/2009 0 20
The top left cell is blank. Select the data and make the area chart. Excel
should recognize the dates and give you a date-scale X axis. If not you can
change that from the Chart menu > Chart Options > Axes dialog (Excel 2003
and earlier) or from the dialog you get when you right click the axis and
choose Format Axis.
- Jon