Average unique count


Is it possible to have the average of unique count of a string field?

For example :
In the document , there is a filed which contains name of store.
I want to have the average number of unique store per hour of day over a complete period.
I know it is possible to have "Date Histogram" with "Hourly" interval but in this situation, I can just have number of unique store instead of average.

P.S : There is a scripted filed which contain " Hour Of Day" .