I don't think I quite got there, but maybe this can give ideas.
Instead of a Gantt, I used the Text Mark to plot the Target.
The label position was given by:
(IF ATTR([Color])="1" THEN SUM([sum(Actual)])
ELSEIF ATTR([Color])="-1" THEN SUM([sum(Target)])
With the 2000 added just to give a buffer.
The target line was demarcated using a reference line.
Please see workbook v18.1 attached in the Forum Thread.
282141label.twbx 22.9 KB
boom! goes the dynamite