Easiest way would just be a calculated column right? You can either do this directly in the table, or on the fly in the graph builder (or whatever other tool you are using) and right click on the column you want to change and select formula, and you can create a quick formula for it.
Alternatively (and this I think is exactly what you want but I find it a bit kludgier), you can create a custom format. So right click on your access and go to the axis setting. Under the format options select Custom, then hit the Set Custom Format button. Then enter the formula for your custom format.