This is a plot of the cumulative distribution of the size of email
messages handled by the UTORmail system. The x axis is the size of the
message in bytes on a logarithmic scale. The y axis is the number of
messages of that size or greater. The dates are the middle Wednesday of
March, July, and November.
The difference between the left side of the curves is just a reflection of the different numbers of messages handled on each date; see next plot. 

Here is the total number of messages delivered on each day. The November and March values are increasing linearly.  
Here are the 10,20,...,90 percentiles (deciles) of the message size
plotted on a logarthmic scale vs time. For example, the bottom line
represents the 10th percentile  10 percent of the messages are this
size or smaller. The middle line, the 50th percentile, is also the median:
half the messages are this size or smaller.
Notes:
