plot roadmap chart in excel
 

I have a data set that consists of multiple groups of data, call them "a",
"b", "c", etc., each with a several milestones ("m1", "m2", "m3", etc.). I
would like to be able to create a table of dates in excel so that when
plotted I get a chart that shows the timeline on the X axis, and the
milestones labeled with their correct named text labels from the table, each
grouped on a horizontal line labeled in the legend according to the group
label.

I've gotten close with a scatterchart by faking the Y values (i.e. the
markers come out correctly), but can't figure out how to assign text labels
automatically from another column in the spreadsheet to the data markers.
Tried a stacked line chart, but couldn't get the lines to separate.

The whole point of this is to construct a roadmap chart for several software
release lines that is easy to regenerate when dates for milestones change.