Extending trend lines in Numbers '09

Unfortunately, Numbers charts don't have an option to extend the line. However, if your trendline is a linear trendline, you can easily add a "forecasted" data point to your series in your table and include it in the plot. Or you can add forecasted points to both ends of your chart if you want to extend the line in both directions. If these forecasted points are outside of the range of your chart (i.e., less than the axis min or greater than the max), the forecasted data points will not plot, just the extended line.

Here is a thread on the subject:

http://discussions.apple.com/thread.jspa?messageID=10012513&#10012513

You can do it for the other types of trendlines by using the trendline equation created by the chart. It isn't quite as good because the equation is usually rounded (i.e., not exact) and if your data changes you have to delete your fake points and re-do them with the new trendline formula.

There are ways to determine the "extended" points of other trendline types with formulas if you are willing to have a few extra columns or a separate table in which to do the math. What type of trendline are you using?