Vertical axis is percentage.
"For example, the theoretical curve shows that 87% of blocks should be found after less that (2 * the current difficulty) shares."
Oops. What I meant to ask was about the Horizontal axis, not vertical. My mistake.
How are the numbers 2,4,6,8,etc determined. I understand the percentage (vertical) line, but I don't understand where the horizontal axis numbers come from.
Thanks for dealing with my noob'ness.
The horizontal axis is multiples of the difficulty. So point 2 on this axis represents a number of shares equal to twice the difficulty. The corresponding point on the vertical axis, 87%, means that 87% of all rounds will end before reaching this number of shares.