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Add symmetric norm for colormaps #1806
Description
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.
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
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phobson commented on Dec 11, 2013
I implemented this another way for my own use:
https://gist.github.com/phobson/7916777
Personally, I think a vcenter kwarg would be great -- especially if it could propagate to all the methods already using vmin and vmax. That sounds like a pretty big effort, however.
tacaswell commented on Jan 24, 2014
@endolith @phobson If either of you want to get a PR together, we can try to get this in 1.4, otherwise, I have tagged this for 1.5
phobson commented on Jan 24, 2014
@tacaswell do you think that simply trying to add a vcenter kwarg to colors.Normalize is a good place to start?
The other question is what would this PR really being trying to accomplish? More specifically, is doing something like this notebook without having to create a totally new colormap the desired end result? http://nbviewer.ipython.org/gist/phobson/7916777
tacaswell commented on Jan 24, 2014
I think the simplest thing to add is @efiring suggestion to, if vcenter in passed in push out which ever of vmin and vmax is closer to make the range symmetric (to basically accomplish the upper-right graph from your notebook).
phobson commented on Jan 25, 2014
@tacaswell taking a stab at this now.
tacaswell commented on Nov 26, 2014
@phobson Did anything come of this?
phobson commented on Nov 26, 2014
I've got an outdated branch on a machine and I could never get all of my edge cases to pass their tests. Basically all I'm trying to do.
Knowing what I know now about MPL's internals, I think a separate subclass would be best and could probably take a look at that tonight. The other option is to add the "vcenter" kwarg to the main Norm class. But that seems like a bad idea. Thoughts?
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On Wed, Nov 26, 2014 at 2:25 PM, Thomas A Caswell
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tacaswell commented on Nov 26, 2014
On one hand, a new class would be the least disruptive change, on the other hand, I think it would basically be the Normalize class with slightly different __init__.
On balance, I think a sub-class with arguments like SymNorm(center, half_width) is probably the best way to go because
phobson commented on Nov 27, 2014
#3858
petehuang commented on Jan 2, 2017
Tracing related PRs, it seems like the most current related work sits in #7294
QuLogic commented on Jul 8, 2021
Implemented in #17709.