Fix # web users graph

fixes http://manage.dimagi.com/default.asp?144599#805971

reimplements a broken query
The problem with coercing to a list is that ```list('abcd') == ['a', 'b', 'c', 'd']```, breaking existing use cases.  Alternatively we could keep just the ```domain``` function and permit either strings or lists of strings as arguments, avoiding introducing a new function.