It's a battle of intuitiveness. On one hand, you've got "steep=hard to climb" association, on the other hand you have "time on the horizontal axis" convention (but "proficiency on the horizontal axis" isn't "wrong", just "less popular"). So, IMHO it's not a matter of "right" vs "wrong" but rather "intuitive" vs "more intuitive".
I think that "steep=hard to climb" will win, because it appeals to anyone who at any point in their life has climbed a stair, as opposed to the x-y curve which even people trained in mathematics sometimes mix up.