Displaying colorbars with specified boundaries correctly

kdpenner · May 10, 2020, 10:16pm

Currently, colorbar takes boundaries and values kwargs. If values is specified, len(values) must be len(boundaries)-1. values can be None. For either case the colorbar displays one color between 2 boundaries. This means colorbars for complex colormaps can be displayed incorrectly.

I’m working on what will eventually become a pull request:

(For more on inconsistent plotting of the red line, see the next post.)

If boundaries is specified and values is not, I uniformly space 1000 values between each set of boundaries to retrieve colors. 1000 is an arbitrary choice—can I make it less arbitrary?

kdpenner · May 10, 2020, 10:17pm

Re: the red line: whether or not it shows up in the colorbar depends on the backend. The above image is from PNG output; here is a screenshot of PDF output:

What’s going on?

jklymak · May 11, 2020, 7:34pm

Can you post the code you are using? Not particularly following what you are trying to do, and what might be going wrong…

kdpenner · May 11, 2020, 8:48pm

You’ll need to grab colorbar.py from my fork:

https://github.com/kdpenner/matplotlib/blob/colorbar-boundaries/lib/matplotlib/colorbar.py

And then run this code:

gist.github.com

https://gist.github.com/kdpenner/5dd18a42d68ea144fcb5aa7e37a1d849

colorbar_example.py

#!/usr/bin/env python -tt

import numpy as np
import matplotlib.colors as colors
import matplotlib.pyplot as plt
import matplotlib.cm as cm

norm = colors.Normalize(vmin = -11643, vmax = 9747)
cdict = {'red': [(0., 0., 0.),
                 (norm(-8000.), 0., 0.),

This file has been truncated. show original

to generate the PNG. Change the last line to get a PDF version.

jklymak · May 11, 2020, 9:21pm

So, are you just letting us know you are working on this or are you asking for something?

kdpenner · May 11, 2020, 11:37pm

As I mentioned, 2 questions:

If boundaries is specified and values is not, I uniformly space 1000 values between each set of boundaries to retrieve colors. 1000 is an arbitrary choice—can I make it less arbitrary?
Re: the red line: whether or not it shows up in the colorbar depends on the backend. What’s going on?

kdpenner · May 19, 2020, 2:19pm

Consistent plotting of the red line requires higher dpi than the default.