I would say add a inline table and do this to get what you want.
Load * Inline [
Metric, MetricVisits, MetricMinutes
Note: Bolded above needs to be exact column names that you have in your data model including capitalization.
In UI create a list box. You can use attached extension which has "Always one selected' value, so that you can avoid senario where user picking both PC&Mobile.
In your expressions use this.
bi-listbox_v2.zip 29.5 K