Graphs the number of active users on a subreddit. I just wrote this. You need add a list of subreddits you want graphed to your munin-node file (on debian/ubuntu it's in /etc/munin/plugin-conf.d/munin-node, probably other places on other distros), like such:
[reddit-usersonsub]
env.subs AdviceANimals,announcements,AskREddit,atheism,aww,bestof,blog,funny,gaming,IAmA,movies,Music,pics,politics,science,technology,todayilearned,videos,worldnews,WTF