Fix CChart scaling edgecases (constant value and infinity)
Reviewed by: vladislavbelov Differential Revision: https://code.wildfiregames.com/D167
See https://github.com/0ad/0ad/commit/c2647d6e853f6e7f14ccc2d4b14e73f0fab40d3c#diff-046d62b1f156cd63a5d67320b254805b.
See #4722
Ah, hadn't noticed that one.
(It was just for reference, don't disagree with getting Imarok to fix it ;) )