The OP is plotting the ratio of page-aligned vs. unaligned access time against problem size. So, on the y-axis, a value of 2 means the page-aligned access took twice as long as the unaligned access.
Took me a few moments to grok the charts too, since I haven't had my coffee yet.
Took me a few moments to grok the charts too, since I haven't had my coffee yet.