(1) When you build a chart using the Chart Wizard, the Wizard gives you
several options to label a heading, X and Y axis, as well as a data labels
option.
(2) If the chart is already built, you can do the same as described in 1
above by clicking on the chart, going to the standard toolbar and clicking on
Chart -> Chart Options.
(3) You can apply labels via the text box option in the drawing toolbar.
(4) You can insert numbers and/or letters by modifying the source range for
the underlying data.
(5) You can apply labels via several free Excel addins that are available at
the following sites: