The average will just be one number (based on the total number of posts since registration). A "burst post" graph could be interesting to show the distribution of time between posts. Say:
-number of posts within less than 2 minutes: x
-number of posts within 2-5 minutes: y
-5-10 minutes: z
-10-30 minutes: a
-30-120 minutes: b
-120-720 minutes: c

You catch my drift