jannette,
The simple answer is, you don't. Project isn't very good when it comes
to graphics capability. You can create an "S" curve, but you will need
to export the data to another application (e.g. Excel).
If you have a version of Project prior to 2007, you can try the "Analyze
timescaled data in Excel" utility found on the "Analysis" toolbar, or
for the most flexibility, you can create your own VBA macro (more on
that later).
If you have Project 2007, you can use the visual reports utility.
Fellow MVP, Rod GIll, wrote a book on Project VBA. One of the examples
in his book is a macro for creating "S" curves. For more information on
Rod's book, go to,
http://www.projectvbabook.com
Hope this helps.
John
Project MVP