in a bar chart i try to draw a gradient from red to green (works fine). But i have no clue how to 'end' the gradient's colorchange related to the data.

store has two relevant fields: name and percent_done.

i'd like to draw a gradient which shows red for 0%, yellow for 50% and green for 100%. so the display for
name1 0
name2 50
name3 100

name1: should be red (but very small)
name2: should be red to yellow
name3: should be red to green

the only thing i get to work is --> all are red to green

any hint please !