There should be possibility to show on one graph two merged periods (e.g. show data for same graph, for same period of time, starting in two different points of time).
In this way you can easily track trends or find abnormal behaviour etc.

This also should be relatively easy to add as a new feature. Please let me know your thoughts.

Also a link the related thread on the forum:
https://www.zabbix.com/forum/showthread.php?t=39148