I have done something similar with Pie Charts, haven't tried it with a bar chart. Below are two ways I have used to create percentages in a Pie Chart. Perhaps you can apply it to your situation.
If the pie chart is placed as a subreport on a main report and the main report can be bound to a query, then you can create a dynamic text box on the main report that points to the value/format you need. Your text box then would be something like:
="Slice 5: " & Format([Slice5],"Percent") & " " & [Slice5]
Which would display:
Slice 5: 25% 300
Of course you would replace Slice 5 with the description, or omit if you don't need and then replace Slice5Total with the actual field name to use. Then apply your formatting, whether PCT Label or Value. You may need to use format masks instead of "Percent" if result doesn't display as you want.
The notes above were from the first time I tried it back in Dec 03. Another method is to put the text boxes on the subreport and refer to the main report because the calculations had to be done a different way than the default that the access chart provides. The text boxes on the main report would need to be in the header section so results probably would need to come from a cross tab as this method would need to work off of one row rather than multiple rows.
=[Reports]![MainReport]![txtFieldData]
