fix for #54, checks for duplicate data keys when drawing ride chart -- adds int duration as param in drawAttendChart

Previously, an `IllegalArgumentException` was occasionally thrown in `drawRideChart` when it was attempted to add multiple rides with the same key. It now checks for duplicates, and if there are any it adds (`count + 1`) to the key name.

A duration arg was also added to `drawAttendChart` to replace the no longer existant `this.maxtime` variable.