Suppose you have a field called MyField. You can create a variable called vField and set it to MyField. You can then create a calculated dimension: $(=vField). By changing the value of vField, you can switch different dimensions.
Similarly, you can accomplish the same thing by creating an island dimension and then picking the desired field. For instance, you could create an island field called AllFields which would contain the names of all fields you'd like to display. If a user selects a single value of AllFields, you can dynamically set that as the chart dimension by creating the following calculated dimension: $(=AllFields)
However, what you cannot do, is dynamically define the number of dimensions. You'd need to create one of these calculated dimensions for every dimension you'd like to display. For instance, if you want your chart to have 3 dimensions, you'd need 3 calculated fields.
Alternatively, if you know in advance that you want only certain fields to be possible dimensions, you can just make them regular dimensions and enable/disable them conditionally on the Dimensions tab.
Hope that helps.
Vlad
