It sounds like you need to a date aggregation level that is not native to the Tableau's time hierarchy, so that you can roll up your data in 30min intervals. Don't worry, this isn't difficult; just write a calculated field with the following formula:
The obvious note is that you should your own date time field in placed of the [Date Time]
When you choose to display this as discrete or continuous exact date, it should only give you data points at the start and middle of each hour. Likewise if there's 6 data points that occur between 1:00AM-1:30AM, these 6 data points will be summarized under the 1:00AM data point for the new time field. Thus taking the avg(Values) on this new time field should effectively give your the avg over 30min intervals.
Hope this helps!
Perfect! Exactly what I was looking for.
Thanks for the quick reply.