In the chart properties | caption dialog, enter text in the help text box. For a chart, this will add a ? to the icons in the caption, which will pop up the help text if you hover over the ?, or click it.
To create a hover pop-up on the entire chart, overlay the chart with a transparent text box, and add the help text to the text box in the same way as above. Now hovering anywhere over the transparent text box will cause the help to pop-up. Make sure that the text box is in a layer above the chart.
Of course, this second approach prevents the user from selecting data from the chart!