These graphs show the percentage of active characters in several level brackets. By active, we mean characters that killed something (creature / player) in the past four weeks.
The first graph shows information about how many characters are actively played at which level currently. It will change over time as it will be updated bi-daily considering character updates in the database.
The second graphs shows how this playing behaviour changes over time. As can be expected, most players seem to concentrate on their max level characters. You can easily spot the moment when Warlords of Draenor was released.