Since r30397, TAChart has TDataPointHintTool that displays a hint about chart points on mouseover.

While implementing it, I encountered a few problems:
1) Hint is only displayed if no control key is pressed (see GetHintInfoAt function), so Shift property of TDataPointHintTool does not work.
2) To display hint immediately, I have to globally set HintPause to 0, possibly interfering with other hints.

I suggest to add ShowHintImmediately (or better named) method, which would display hint regardless of the control keys state and without pause.