… string from the name_attr() and level_attr() functions.

…f the max value. Had a less-than sign that should have been greater-than. Also, switch from tracking data by it's magic_str() to simply using VAR_i and VAR_j strings. We *could* have just used the i, j integers directly, but constructing the ij joint histogram integer could have been tricky since we start numbering with 0 instead of 1. i=0, j=1 would result in 01 which is the same as integer of 1. If we do want to switch to integers, we just need to make them 1-based and add +1 all over the place.