Simplify test for negative group, which also fixes the problem of the "flipping" transform when hiding the last source within a group.
Wrap in function for strictness.
Remove class-level variable that is only needed within local function.
And function wrapper for strictness.
Remove unused code.
Make y-axis ticks use value for d3 key, so transitions are smoother.
Remove old unused code.
Update @require annotations.
Migrate power-io chart to general node datum API.