Actually I think that graph is a horrible representation of what most people are trying to say.
If skill is measured out of 10 (with 10 being the best resort riders you've ever seen, and a 3 being able to at least once a day make it down a blue run without falling)...
Then in general I'd say skiers can be at a level 3 much quicker than boarders, but boarders after the 3-5 days off ass and knee pain would get up higher than that quicker.
A friend of mine just left today, she was visiting from Toronto and she's both a skier (intermediate) and boarder (noob). She left her board in Toronto because she didn't want to slow anybody down or limit herself in the runs she could do at Fernie. Turns out she was bored after a few runs on her skis and was talking about renting a board for the second day. I took her out myself on day 2 and showed her more of the mountain and I think she had a lot more fun, but her gut reaction was still that she found skiing boring and snowboarding more of a challenge.
I know after 21 years of boarding I can go anywhere a skier can, and I don't find boarding boring at all! There's always something more to improve on, and I feel like you hit this wall with skiing where unless you're starting to huck yourself off lips or do freestyle tricks, it's a little too easy to just carve around the hill...