Label points on a line chart with a simple string (i.e. A)

I need to be able to label certain points on a line chart with a string single letter label so I can refer the user to point A, point B and so on. Do I need to look at NSFormatter or is there a simpler way?

The issue identifies line-chart point labels and asks whether NSFormatter is involved, but names no source file or test. Start by locating the line-chart label or formatter entry point, then determine how selected points can receive single-letter strings and verify that those labels render as requested.

Written by the indexing model from the issue text.