@ Mechatrommer:
I guess I was 2 minutes late; hey, great minds work together.
i got sleepy when bending my neck 90 degree, but what i'm aware of is your image is up when i hit the post button so i guess i'm the 2 minutes late one. but nevermind i wont bother to correct my graph "de-offsetted" y range. i think yours is more corrected graph. and more detaily explanation will be needed if OP havent attend secondary class topic like ratio, percentage and how area of trapezium is derived.
i should have make it clearer
(x - a) / (y% - 0%) = (b - a) / (100% - 0%) ... slope or ratio formula... but well...