[FR] symmetric contour levels, the ability to ignore the actual values.

[ryofurue]

I often need to plot positive and negative values with reddish and bluish colors because the signs are important; also I need to respect the magnitude (I mean, for example, "3" and "-3" should have red and blue colors with the "same" "strength". There are well designed color palettes like redsblues suitable for this task, except that the contour() function doesn't allow the user to specify symmetric contour levels:

using Plots
xs=-5:1:5
ys=-5:1:5
var = [ x^2 + y^2 - 15.0 for x in xs, y in ys]
p = contourf(xs, ys, var; color=:redsblues, levels=-40:10:40)
display(p)

Here I want the contour levels to be [-40, -30, . . ., 0, . . . , 30, 40] but the contour function overrides my wish and adjusts the contour levels to the actual value range of the variable, which is -15 < z < 35.

[BeastyBlacksmith]

Setting clims = (-40,40) should give you that

[ryofurue]

Setting clims = (-40,40) should give you that

Indeed! Thanks, but I didn't mean to use this issue tracker as if it were a help forum!

But, this indicates that there is another bug. Let's use this modified example:

using Plots
#plotly() # GR doesn't support open ended contour levels
xs=-5:1:5
ys=-5:1:5
var = [ x^2 + y^2 - 15.0 for x in xs, y in ys]
#p = contourf(xs, ys, var; color=:redsblues, levels=-20:10:20)
p = contourf(xs, ys, var; color=:redsblues, clims=(-20,20) )
display(p)

With GR,

1. lev=-20:10:20 crashes the program.
2. clims=(-20,20) gives a correct plot with symmetric contour levels, except the colorbar indicates as if only the range -20 < z < 20 were plotted, whereas the contour plot itself paints values beyond ±20. The colorbar should indicate the open-ended-ness of the contour levels.

I think I reported issue 1 as a separate bug. Should I report issue 2 as well?

With Plotly,

1. lev=-20:10:20 creates a correct plot with symmetric contour levels. The colorbar indicates the open-ended-ness of the levels.
2. clims=(-20,20) gives a similarly correct plot as the lev example.

So, I guess that GR should be modified to behave exactly like Plotly in this regard?

[BeastyBlacksmith]

Should I report issue 2 as well?

I'd say that is pretty much the same as #4805, no?

[ryofurue]

Should I report issue 2 as well?

I'd say that is pretty much the same as #4805, no?

I don't think so. #4805 is just a "nice features to have". It's not a bug.

But issue 2 above is a bug. The following plot indicates that all the values are within the range -20 < z < 20:

using Plots
xs=-5:1:5
ys=-5:1:5
var = [ x^2 + y^2 - 15.0 for x in xs, y in ys]
p = contourf(xs, ys, var; color=:redsblues, clims=(-20,20) )
display(p)

The fact is the variable includes values larger than 20. If you compare the plot and the colarbar, you'll see that the color for 15 < z < 20 is used to plot z > 20. I would say this colorbar is incorrect.

For this reason, this plot cannot be used in a scientific paper.

[ryofurue]

So, I've just created a new issue: #4813