I have created a graph/chart.

Some data points are outliers and I would like to label these with why they are
outside the norm. I know how to individually select a data point and give it a
label (click on the series, Ctrl+click a point, Insert Data Label), but it seems
that I can only have it display the value for that point and not write custom text.

I can draw a text box with a custom label, but whenever I change the underlying
data or add more points, this will be in the wrong place. These "fake" labels
are how I have created the mock-up.

The easiest way that I can think to do this would be to have an additional
option in the Format Single Data Label dialog box for Custom Text (along with
the Number and Percentage options).