The `aspect_ratio` controls the digit spacing, ignoring the units

[JeffFessler]

The following MWE makes a 2inch by 2inch square, using a mix of inches units and mm units for the two axes.

using Unitful: mm, inch
using UnitfulRecipes
using Plots

x = LinRange(-4,4,201) * 10mm
y = LinRange(-20,20,203) * inch

z = @. (abs(x) < 1inch) * (abs(y) < 1inch)'

ar = [1, :auto, :equal, :none]
pl = Array{Any}(undef, 4)
for (i, aspect_ratio) in enumerate(ar)
    pl[i] = heatmap(x, y, z'; aspect_ratio, title="$aspect_ratio", 
        colorbar=:none, alpha=0.5, 
        xlabel="x", ylabel="y", ytick=(-1:1)*20inch)
end
plot(pl...)

However, all four options for aspect_ratio, i.e., [1, :auto, :equal, :none] produce a rectangle rather than a square, because 1 and equal make the spacing of the digits equal, rather than the physical units. (Note the grid lines make squares for 1 and equal.)

Here is the resulting plot using UR v1.5.3 and Plots v1.13.1:

In the long run, it might be preferable to have some option (perhaps equal) for aspect_ratio that makes the actual distances match (when the units are compatible). I have no idea whether it can be done here in UR or would require downstream changes in Plots.

In the short run, this issue at least will document the behavior. It would be easy to at least make the current heatmap example more informative by having axes with units:

https://jw3126.github.io/UnitfulRecipes.jl/stable/examples/2_Plots/#Heatmap,-categorical-axes,-and-aspect_ratio

[briochemc]

This seems like a convoluted corner case to me that might not be worth the effort to fix, but maybe it's just me? 🤷‍♂️

Why would one use different units if they intend to set the aspect ratio to equal? Should this throw when the units of the x and y axes are incompatible?

[JeffFessler]

In the actual situation where I hit on this, my units happened to be mm and cm, which seems less convoluted than using both inches and cm. I am sure there are more important issues to fix elsewhere, but I just wanted to document it. Personally I would prefer at most @warn rather than throw for incompatible units.

[briochemc]

Personally I would prefer at most @warn rather than throw for incompatible units.

Ah I would have thought you'd want it to throw in this case...

Anyway, I think you are right: It's good to document this! Maybe we can add a warning in the docs to say that aspect_ratio works on unit-stripped values rather than unitful values?

[gustaphe]

In theory, the aspect ratio for a unitful plot should be a unitful number (Hz/cm), with :equal corresponding to 1.

[JeffFessler]

Agreed, so now I also agree that :equal should throw for incompatible units. I might still wish for a different option name to handle cm/mm with physically meaningful aspect ratio but that's probably not worth the effort. Just having a warning in the docs seems OK.

[gustaphe]

Closed in #77