Plots.jl: [BUG] Fill between lines using `ribbon` doesn't work on GR, Plotly

[jdossgollin]

Details

I've been trying to replicate thefill_between in PyPlot using the ribbon argument in Plots.plot. It works great (actually I like the fill_between API where the user specifies the upper and lower lines, not a center line and delta above and below it, but that's a side point). For GR and Plotly, however, the fill doesn't show up in the legend of a plot. I think this is a bug.

using Plots

function make_plot(backend_fn)
    backend_fn()
    x_plot = 0:0.05:6.5
    lb = sin.(x_plot) .- 1
    ub = sin.(x_plot) .+ 1
    fake_middle = lb + (ub - lb) / 2
    p = plot(
        x_plot,
        fake_middle,
        ribbon=[ub .- fake_middle, fake_middle .- lb],
        alpha=0,
        color=:gray,
        label="Fill Label",
        title="$backend_fn",
    )
    # these work
    plot!(p, x_plot, lb, label="Lower Bound")
    plot!(p, x_plot, ub, label="Upper Bound")
    return p
end

plots = [make_plot(b) for b in [gr, pgfplotsx, plotly, plotlyjs, pyplot]]

Backends

This bug occurs on ( insert x below )

Backend | yes | no | untested
-------------|-----|-----|---------
gr (default) | x | |
pyplot | | |
plotly | x | |
plotlyjs | | |
pgfplotsx | | x |
inspectdr | | | x

Versions

julia> versioninfo()
Julia Version 1.5.3
Commit 788b2c77c1 (2020-11-09 13:37 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin18.7.0)
  CPU: Intel(R) Core(TM) i5-5287U CPU @ 2.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, broadwell)

Plots 1.6.7

Output

[BeastyBlacksmith]

If you want to specify upper and lower curve you should use fillrange

x_plot = 0:0.05:6.5
plot(x_plot, sin.(x_plot) .+ 1, fillrange=sin.(x_plot) .- 1, fillcolor = :gray)
plot!(x_plot, sin.(x_plot) .- 1)

[jdossgollin]

Cool, that works and shows up on the legend!
(At least using GR, haven't tested plotly)