[help!!!]How to display specified time in the middle of the chart?

I want to display specified time in the middle of the chart. Is there any function to realize it?

You'll need to code something like that using either time_current or time_local. Then display with something like Comment.
ubzen:
You'll need to code something like that using either time_current or time_local. Then display with something like Comment.

I don't get it. For example, I want to display yesterday's data in the chart automaticly. How could I do it with some code?