small fix for the placement of the zero line.
Merge branch 'master' of github.com:heavysixer/d4
refactoring the parsers some, and adding tests for the nested stack parser.
prevent developers from using `y` as a key value for the x dimension in a nested stack because it creates an ambiguous y value that could belong to either the stack itself of the x dimension. The waterfall and nested group parsers may also suffer from the same problem.