Capacity Trending Graph Help

So, I started creating a SQL query to attempt this.

I joined a few tables as such:

SELECT SRM_StorageArrayCapacityStatistics.CapacityUserTotal, SRM_StorageArrayCapacityStatistics.CapacityUserUsed, SRM_PoolCapacityStatistics.CapacitySubscribed, SRM_PoolCapacityStatistics.DateTime, UserCaption

FROM SRM_StorageArrayCapacityStatistics

FULL OUTER JOIN SRM_PoolCapacityStatistics on SRM_StorageArrayCapacityStatistics.DateTime = SRM_PoolCapacityStatistics.DateTime

FULL OUTER JOIN SRM_StorageArrays on SRM_StorageArrayCapacityStatistics.StorageArrayID =SRM_StorageArrays .StorageArrayID

WHERE year(SRM_StorageArrayCapacityStatistics.DateTime) =Year(getdate())

I see the following output in table form so I am getting some data:

In Reports Manager however, when creating a custom chart with this query, I see the error message below. I thought I could group data by UserCaption (System name). For some reason it doesn't like this.  Any ideas? If I can get the basic output in the graph, I can customize further.