I think it should show the longest up and the longest down time so if the cue contains a slow colour fade and a slow dimmer fade then it would show the colour. I think that this is important not to show just the dimmer as when using LEDs and want to cross fade from Red to Blue, this will be shown even if there is no dimmer change.
As a second idea, each bar could be split into two, one for dimmer up (Green) and one for longest Attribute fade (Blue) on the UP times. The DOWN times could show dimmer down (red) and longest Atribute (Blue again?).
So would 'time points/markers' be able to be shown on the bar? Maybe to much info if all the secs and mins were shown but maybe 25% compleate, 50% compleate, 75% etc.