If I understand correctly, I think a query like this should get you started:
SELECT DATETRUNC('month',rt.DateTime) AS d, n.Caption, AVG(rt.Availability) AS Avail
FROM Orion.ResponseTime rt
INNER JOIN Orion.Nodes n ON rt.NodeID=n.NodeID
WHERE n.CustomProperties.Department = 'Enterprise Servers'
GROUP BY n.Caption, DATETRUNC('month',rt.DateTime)
ORDER BY d, n.Caption
In PowerShell, you can fetch this data using "Get-SwisData $swis $query | Export-Csv -Path myreport.csv". You can then load this into Excel for formatting and charting.
I finally got around to testing this. Where in this statement is it telling it to pull the average data for the last 12 months?
That will pull all the availability data in the database (with no limit), grouped by month. If just want to limit it to last 12 months, add "AND rt.DateTime > ADDYEAR(GETUTCDATE(), -1)" to the WHERE clause.
If you need something else, please be specific about what you are looking for in terms of grouping and aggregation.