I assume GP is talking the world in general, but it was indeed "unthinkable in the not so recent past" even for Iceland: for example, in 1975 (which I guess you'll agree as being "not-so-recent") there were only three women parliamentarians.
Politicians doesn't change overnight as barriers are removed. A man who entered the field in his 20's 1975 got retired just about now, that is how long it takes for change to fully propagate throughout society. So if 1975 was the year that it got equally acceptable for men and women to enter the field now is about the time you'd expect men and women to have equal seats in politics.
And that graph lacks the last election result, Sweden currently has 47% female parliament members, so about even by now.
Edit: Looks like USA is about where Sweden was 35 years ago on this. Reminds me how different cultures can be, and why the American gender discussions are so different from what I'm used to. So I guess USA might see a gender equal parliament in about 35-40 years.
