Hi Farshad!
I think need to include "summaryType":
count
sum
min
max
average
http://www.objis.com/formationextjs/lib/extjs-4.0.0/docs/api/Ext.grid.feature.Summary.html
If I'm not mistaken there is currently no such possibility (of course, we can use the "summaryType" in code, in another way. But would like that this possibility was in the properties of the column)
Sincerely.