I created a trivial patch to use what I think are more readable date formats in graphs.

Current issues:

1) It should be configurable and not just hardcoded differently that the current format
2) It may need to be language localized since it uses month and day names
3) Graphs are not currently re-sized for the larger text length

Please see discussion forum: http://www.zabbix.com/forum/showthread.php?p=60192