So to just make clear, these charts only show the maximum concurrently connected players. Not how many people connected that day just the amount of people who were online at the same time per hour and per day.

You may notice in the 24 day chart that about 16-17 days ago the amount of concurrently connected players dropped hugely. This was when Minecraft 1.5.2 launched and we were still running a 1.5.1 server. It returned to normal the following day after I updated us.
Overall I'd say that the stats are positive, the server is definitely growing a stable following and at this time of year this is quite good. During the summer when students leave for the summer holidays I wouldn't be surprised to see us reach 30 concurrently logged in users several times a month.