Missing "Value from Cells"" option when formatting data labels on scatter graph on Mac

I have drawn a cube in perspective using a scatter graph. Each of the 12 edges of the cube is a separate 2-value data series. I expected to be able to label the cube's vertices by adding data labels to 4 of the data series (e.g. the ones corresponding to the vertical edges).

I can select an edge and add data labels. When I try to format the labels, the "Value from Cells" option is missing. I want to label the vertex as a letter (selected from a separate column) not using its x or y coordinate value or series name.

Is this a "feature" of Excel for Mac? Or should I be able to do this?