Trendline through the origin

I may be missing you point, but do you have a data point at 0,0, and is there any reason to expect that the estimate should pass through 0,0? If yes to both, then you might consider using a polynomial fit. Keep raising the order of the polynomial until the line comes as close to your actual data point as you want, but beware that higher orders can create pretty bizarre curves that probably don't represent reality.

Jerry

You could add a second series, with a data point at 0,0, and call it a trend line. This would require that you calculate the slope yourself and use it to determine the location of the other end of the line.

Jerry

You cannot force a true trend line to go through zero - it can only do this if the source data enables it to go through zero. The only way would be to add numerous false data points at 0,0 to bias the trend. Not very scientific!

See this thread

http://discussions.apple.com/thread.jspa?messageID=10626015&#10626015

You will have to create the trendline as a series in your table.