We did in Enterprise.
Well, sort of. The cool thing about that graph is that it appears to establish that the speed associated with a specific warp factor can vary by a whole order of magnitude. Depending on what factors, we don't know, but the alternate curves and blue "error bars" in that graph offer enough variety to accommodate every single datapoint ever dreamed up by the writers, let alone the few that actually made it to the screen.
Timo Saloniemi