More than likely the trendline feature is dimmed due to the type of chart
you're using. You can add trendlines to data series in unstacked 2-D area,
bar, column, line, stock, xy scatter, and bubble charts. You cannot add
trendlines to data series in 3-D, stacked, radar, pie, surface, or doughnut
charts. Certain combinations of the above will probably not allow trendlines
either. If your chart type is included in one of the types that does not
accept trendlines, I would suggest changing it to one that does.
Alternatively, you could calculate the trendline in worksheet cells and then
plot those cells as a seperate series. The trendline formulas are available
via Excel's on-line help.
