We need possibility to have semi-transparent colors on Graphs. If you look at Cacti you will notice by example for a CPU graph is possible to use a stacked graph and idle CPU is with a semitransparent color.

This is looking nice because you can still see the grid lines from behind and can get an easier estimation related to CPU usage (without idle CPU).

This is currently not possible on Cacti it seems. There is not possible to assign a transparency level to images.