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How to change the median value on colorbar

Asked by Vince Clementi on 26 Sep 2017
Latest activity Commented on by Vince Clementi on 29 Sep 2017
Hi All,
I am looking to adjust the linearity of my colorbar by changing the value of the median value For example, if colorbar ranges from [-6 6], the median is 0. I want to shift it to +1 or -1).
Is there a one-line solution to this or would I need to create a customized colorbar? If the latter, how would I go about doing that?
Thanks.

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1 Answer

Answer by Chad Greene
on 26 Sep 2017
 Accepted Answer

Hi Vince. The answer is you can do this, but you probably don't want to. I think it would mean developing a mathematical transfer function to scale your C data, plot the scaled data, then label the yticks on the colorbar, which will be scaled, but use labels that indicate the unscaled values.
For a simple example, here's what I mean, but simply scaling by a factor of two:
Z = peaks;
figure
subplot(1,2,1)
imagesc(Z)
caxis([-6 6])
colorbar
Z2 = 2*Z;
subplot(1,2,2)
imagesc(Z2)
caxis([-6 6]*2)
cb = colorbar;
set(cb,'ytick',[-12 0 12]','yticklabel',[-6 0 6]')
That works, but it's overly complicated even for a simple scaling without moving the central value around. What you are proposing seems more complicated, more prone to error, and less intuitive for the viewer to understand. In general, if you ever find yourself wanting to do such funny things with the colorbar, or if you find yourself inventing complicated colormaps to show what you want to show, it's probably a good time to step back and rethink what you're doing.

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For example, the first image is the default setting. Notice how the divergent point is centered at the mean of the data range. The second image is after I try to pivot the diverging point to -1.1 and 1.1 in the top and bottom plots, respectively. Any thoughts?
It would be a easier to diagnose if you could provide a minimal working example. I wonder if you're getting the +/- sign of the pivot value mixed up? For example, here's something similar to your top plot using the default settings (pivot value is the center):
imagesc(peaks(500)/12-1.2)
caxis([-1.8 -0.4])
colorbar
cmocean('bal')
Now try this, which is what I think you want:
cmocean('bal','pivot',-1.1)
But I wonder if this is what you did instead:
cmocean('bal','pivot',1.1)
I tried switching the +/- on the pivot mark, but now I'm thinking it's because I haven't set colorbar limits--I let it set itself based on the data. Here's an example of the code (without changing the pivot point):
cmocean('-balance',75);
pcolor(T,Z,dd18o);
shading interp;
set(gca,'YDir','reverse','XMinorTick','on');
colorbar;
c = colorbar;
title(c,['(' char(8240) ')'],'FontSize',12);

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