Using minutes instead of seconds when formatting times for memory usage chart

Hey,

  I noticed some inconsistency around the times on the X-axis in the memory usage chart.  I believe this is because the time is being formatted as Hours:Seconds rather than Hours:Minutes.  This patch changes the format.

Dan

 
![Screen Shot 2013-04-14 at 3 25 15 PM](https://f.cloud.github.com/assets/53167/378683/90e5d892-a553-11e2-82c2-7099bac9facf.png)