Flip function

[hadley]

Should be able to flip vertically or horizontally.

[hadley]

Is there a base equivalent of this? I guess you normally just use reverse indexing - but this would still be useful. (And would just be implemented in terms of matrix operations).

[kohske]

I never find this in a base matrix. Yes, reverse indexing will do this work in the case of base matrix.
In the case of gtable, a little bit more work are required (e.g., modification in layout data.frame).