Distribution of journal publications in different fields. (from Nature). Click on the image to see a larger version.
Notice that parenthetical comment at the top of the figure describing where the data come from -- "as indexed by Web of Science". You'd probably get a similar result if you used Scopus as the source of your data, but I have to wonder whether the proportion of open access titles is larger or smaller among the universe of journals not indexed in Web of Science. Could this survey understate the extent to which open access journals are used. I presume that if you were to count papers posted on arXiv as published, for example, the open access wedge in physics would get a lot larger.
1Stay tuned. There's a post scheduled for Monday morning that will tell you a little more about that trip -- if you're interested.