OpsCenter Report for Frozen Media

This should come pretty close to what you're asking for - 

select t1.snapTime, t1.mediaStatusName, t1.mediaTypeName, t1.LibraryName, count(t1.mediaId) as CountOfMedia
from (
    select UTCBigINTToNomTime(x.snapshotTime) as snapTime, 
        case 
            when lower(x.volumePoolName) = 'scratch' then 'Scratch'
            when lower((select name from lookup_mediaStatus where id = x.status)) like 'active%' 
                 then 'Active'
            when lower((select name from lookup_mediaStatus where id = x.status)) like 'full%' 
                 then 'Full'
            when lower((select name from lookup_mediaStatus where id = x.status)) like '%frozen%'
                 then 'Frozen'
            when lower((select name from lookup_mediaStatus where id = x.status)) like '%suspend%'
                 then 'Frozen'
            else 'Unknown: ' || (select name from lookup_mediaStatus where id = x.status)
        end as mediaStatusName,
        (select identifier from lookup_mediaType where y.type = id) as MediaTypeName,
        z.libraryAlias as LibraryName,
        x.id as MediaID
    from domain_MediaHistory x left join domain_Media y on x.id = y.id, domain_TapeLibrary z
    where x.librarySlotNumber != -1
    and z.id = x.libraryId
    ) t1
group by t1.snapTime, t1.mediaStatusName, t1.mediaTypeName, t1.LibraryName
order by t1.snapTime desc

To explain a bit, every day at (default) midnight OpsCenter collects a snapshot of all your tapes and their current status and attributes. What this query is doing is summing up the tapes in different statuses for each day so you get data like this: 

snapTime	mediaStatusName	mediaTypeName	LibraryName	CountOfMedia
07/27/15	Full	hcart	A	6001
07/27/15	Active	hcart	A	591
07/27/15	Frozen	hcart	A	120
07/27/15	Scratch	hcart3	B	960
07/26/15	Full	hcart	A	5958
07/26/15	Active	hcart	A	596
07/26/15	Frozen	hcart	A	120
07/26/15	Scratch	hcart	A	38
07/26/15	Scratch	hcart3	B	960

Then you can take that data to Excel and graph it, I'd recommend the Stacked Area chart. This will show you exactly when your frozen media is getting high, and when you unfreeze some it will show that too. 

This query is not for 3months, its for however long you have the data for. You can check/change how long your OpsCenter keeps media data by looking at Settings > Configuration > Data Purge tab > Media History field. You may need to tweak this to fit your needs, but hopefully this helps.