I found a couple other posts asking similar questions. Responders came to similar conclusions:
what does the 'm' unit in munin mean?
I'm using version 1.4.5 of munin (vs. 1.4.4), and I saw a max of 400 m accesses/second, which makes little sense. It definitely isn't 400 million accesses per second. 400 x 10**-3 makes sense for what I'm running (a very lightly used web service in our engineering group).
When the value to be graphed is less than 1 unit-per-time-interval, showing it this way is very misleading. If the max value in the graph is less than one, then change the units to accesses per minute, or even accesses per hour if necessary. Or make the max y be "1 access per second" and let the data rumble around in the lower region of the graph.
If no reasonably compact, clearer method of displaying the data can be figured out, I'd suggest a note or tooltip or something that helps the user understand the variable and its units. When one drills down (click through to the 4 graphs day/week/month/year), the variable is explained as: "The number of accesses (pages and other items served) globaly on the Apache server". Add a discussion of units (and spell globally correctly).
Note - I have not checked this on a newer version of munin. Perhaps it has been improved. However, I was not able to find mention of the topic on the http://munin-monitoring.org/ site. Apologies for not going and filing a feature request on munin.