If you are talking about dimensions, you need to format them on the Dimensions tab or in the load statement.
But I'm guessing you are talking about expressions, then you can check your expressions format in the "Number" tab in the chart properties, selecting, for example, "Money" which will display currency and two decimals.
Hope that helps
I am asking about formatting under 'Text Format' tab in Used Dimensions.
My question is what expression would I enter to format the values. E.g. I would enter ='<B>' to make the data bold but I am after an expression to make the format pattern "Money" £#,##0.00 as you would for an expression under number.
As far as I know, the only valid values for "Text Format" are B, K, S, U, even as a result of an expression. That's actually why the Number tab exists. Note that you can change the format pattern in the properties, so, for example, you might use two different currency signs for two different expressions.
So you may use Fabio's suggestion, adding a calculated dimension or create a new field already formatted for your values to be displayed properly in the chart.