return time period in graph title

Previously, the selected time period was displayed in the graph title. This is lost now in pre-4.0.

Showing it in the graph itself was highly useful, as we tend to attach graphs to incidents or provide them to other people for different purposes.
It helped to quickly identify a graph covering 1 hour, month or year. Determining this from the timeline is error prone and is extremely hard.

For example long time ago there was very similar case ZBX-1120.