If you have an expression in a pivot table or straight chart then you can choose minichart as representation and then change the settings for the minichart.
So... are you trying to do this in a pivot chart or straight chart? Other chart types don't allow minicharts
And are you trying to do this for an expression? You can't show a dimension as a minichart.
If this doesn't help, please post the document you're working on (or a small sample)