There was previously discussion about this but I don't think it ever happened:

Yes, that is exactly what I have in mind. I have a branch started to do this, but it is not ready yet. I am thinking of a kwarg called "vmid" to go with "vmin" and "vmax", the idea being that if it is not None then it expands any specified or autoscaled range in the manner you describe.

#875 (comment)

Just wanted to note here that I have a SymmetricNorm class which auto-scales evenly around a centre point.
It sounds like it could do the trick (and potentially avoid mutually exclusive keywords) and I'd be happy to share.

https://github.com/SciTools/iris/blob/master/lib/iris/palette.py#L164