[FR] Ability to resize the arrow heads in e.g. quiver plots

[TorkelE]

I have been trying to figure out how to do this (trying tons of various options), but currently does not seem to be possible in Plots.

It would be useful to be able to resize the arrowheads in e.g. quiver plots. E.g. for

using Plots
xs = vcat([fill(i,10) for i in 1:10]...)
ys = vcat(fill(1:10,10)...)
dxs = (0.8 .- rand(100)) ./2
dys = (0.6 .- rand(100)) ./2
quiver(xs, ys; quiver=(dxs,dys))

Especially when the arrow lines are small, the arrowheads looks really weird and ugly.

[j-adel]

I really suggest this arrow!() function. This issue has been there for over 7 years. I think it's time...

[BeastyBlacksmith]

You are welcome to open a PR 🙂

[j-adel]

You are welcome to open a PR 🙂

It seems to me that function quiver_using_arrows(plotattributes::AKW) is already designed to have scaling arrow heads. I'm not sure if there's something preventing its use or if there's a way for the user to select it.

[BeastyBlacksmith]

True, but that is only used, if the backend supports the :arrow attribute

@recipe function f(::Type{Val{:quiver}}, x, y, z)  # COV_EXCL_LINE
    @nospecialize
    if :arrow in supported_attrs()
        quiver_using_arrows(plotattributes)
    else
        quiver_using_hack(plotattributes)
    end
    ()
end

which GR currently does not:

Plots.jl/PlotsBase/ext/GRExt.jl

Line 25 in da79316

const _gr_attrs = PlotsBase.merge_with_base_supported([

That is why it uses the other implementation

[j-adel]

The problem isn't in quiver_using_hack (which is actually not triggered in GR. In fact, hack has scaling arrow heads:

It's the quiver_using_arrows() that's problematic. But I really don't understand the library structure to see where the line vectors are being computed.

[BeastyBlacksmith]

Okay, so the intended API from the Plots side is to be able to call e.g.

quiver(xs, ys; quiver=(dxs,dys), arrow = arrow(0.5))
quiver(xs, ys; quiver=(dxs,dys), arrow = arrow(0.5, 0.3))

To set the style of the arrows.

The arrow properties are specified by an Arrow stuct, defined here:

Plots.jl/PlotsBase/src/Arrows.jl

Line 8 in da79316

struct Arrow

However, the GR backend currently only uses arrow.side and arrow.style when drawing arrows, like here:

Plots.jl/PlotsBase/ext/GRExt.jl

Line 2024 in da79316

arrowside, arrowstyle = if (arrow = series[:arrow]) isa Arrow

So, IMO the backend would need to be changed to take arrow.headlength and arrow.headwidth into account.

[j-adel]

I see. But it seems like the function utilizes GR.drawarrow() which doesn't take more than the coordinates, the direction, and a preset style.

https://github.com/JuliaPlots/Plots.jl/blob/da79316e5bd768e417f65f344b7d749e41bbb0be/PlotsBase/ext/GRExt.jl#L391:392

From the GR backend https://gr-framework.org/_modules/gr.html#setarrowstyle, it seems like you can't set the arrow head to automatically scale. But there is also a setarrowsize(size). Maybe that's worth trying? I'm sorry but I'm busy these couple of weeks so I can't dive into GR backend at this point.