I have created a bar graph from a pivot table that shows the number of
events that occurred on specific dates, grouped by month. When I right
click the series to add a trendline the 'Add Trendline' option is grayed
out. Why is this?

Excel 2003.

The original data is in the form of a single column of data, each row
contains only a single date. All dates are valid and there are no nulls in
the data set.