How much do figures need to differ to avoid copyright claims?
In general, I would argue that the new plot needs to do one of two things:


present materially different content relative to the old image
present the same material in a different context.


Graphs are generally visual representations of numerical data. The numerical data itself cannot be copyrighted (because the data are "facts"), only the presentations thereof. However, simply changing axis labels or colors, and other similar "gloss" changes, don't really make the graph any different. Rotating a 3D graph, or changing significantly the relationship of blocks in a mind map or diagram to show off different features would represent a change in the message and presentation, and therefore would be OK.

However, chemical structures and mathematical equations are not normally considered subject to copyright, as they represent "facts," and cannot be arbitrarily drawn or represented; there's only so many "legal" ways to write them. That said, cutting and pasting somebody's figure from another article would be a copyright violation; recreating it yourself would not. (Otherwise, we would never be able to write E = mc^2 without it being a copyright violation!)

If there are doubts, however, you can always try to contact the journal in question. If needed, you can ask for permission to reuse a figure, especially if it's one you've already created. Most journals already permit self reuse, so long as you include an appropriate credit and citation of the original.