Instead of implementing my own I was wondering if anyone knows of a histogram or bag datastructure implementation in Objective-C that I can use.
Essentially a histogram is a hashmap of lists where the lists contain values that relate to their hash entry. A good example is a histogram of supermarket items where you place each group of items dairy, meat, canned goods in their own bag. You can then very easily access each group of items according to their type.
NSCountedSet is a multiset (aka "bag") that counts distinct objects, but doesn't allow duplicates. However, based on your explanation, I don't think that's what you need, and neither is a histogram, which automatically buckets values based on a set of (usually numerical) ranges.
I believe what you really want is a multimap, which is a "key to one-or-more values" relation. The data structures framework I maintain includes CHMultiDictionary, a multimap implementation. I won't claim by any means that it's perfect or complete, but I hope it may be helpful for your problem.