a: I don't know about all charts (you didn't specify), but here's how you can attach text (not a text box) to a data point in an xy chart.
1) Select the plotted series.
2) Go to Format Selected Data Series
3) Select the Data Labels Tab
4) Click on Show Label or Show Value Then OK
The labels should appear next to the points
5) Click on the labels (all labels will be selected)
6) Click on the particular label you want text for
(it will then be selected individually)
7) In the formula bar, type an equal sign
8) Go to the spreadsheet and click on the cell that contains the text you want to appear
9) Hit Enter or click the check.
This will put the text next to the data point.
There is a problem with this however in that the text length is very limited and begins to wrap after only a few characters. If you find a fix for this, let me know.