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A general description is here.
plot(sin, -5, 5) gives ylab 'sin(x)' again, where it resulted in 'x(x)' in 2.6.0. Further, plot(sin) again plots from [0,1] also in cases where a previously used coordinate system differs.
curve() with unspecified 'from', 'to' and 'xlim' now reuses the previous x limits, and not slightly larger ones.