Hello,

I’m experiencing the following behavior, which confuses me a lot.

First of all, let’s consider the following code:

```
using Plots
x = range(0, 10, length=100)
y1 = @. exp(-0.1x) * cos(4x)
y2 = @. exp(-0.3x) * cos(4x)
y3 = @. exp(-0.1x)
y4 = @. exp(-0.3x)
y = [y1 y2 y3 y4]
p1 = plot(x, y)
p2 = plot(x, y, title="Title 2", lw=3)
p3 = scatter(x, y, ms=2, ma=0.5, xlabel="xlabel 3")
p4 = scatter(x, y, title="Title 4", ms=2, ma=0.2)
pp = [p1,p2,p3,p4]
plot(pp..., layout=(2,2), legend=false)
```

In this case I put all plots in a vector and splat them into plot, it is not very smart but it works.

In the following I do the same except that I append each subplot to `pp`

instead of constructing it at the end

```
using Plots
x = range(0, 10, length=100)
y1 = @. exp(-0.1x) * cos(4x)
y2 = @. exp(-0.3x) * cos(4x)
y3 = @. exp(-0.1x)
y4 = @. exp(-0.3x)
y = [y1 y2 y3 y4]
pp = []
p1 = plot(x, y)
append!(pp,p1)
p2 = plot(x, y, title="Title 2", lw=3)
append!(pp,p2)
p3 = scatter(x, y, ms=2, ma=0.5, xlabel="xlabel 3")
append!(pp,p3)
p4 = scatter(x, y, title="Title 4", ms=2, ma=0.2)
append!(pp,p4)
plot(pp..., layout=(2,2), legend=false)
```

Here I get the error

```
julia> show(err)
1-element ExceptionStack:
LoadError: Couldn't process recipe args: (Plots.PlaceHolder, Plots.PlaceHolder, Plots.Subplot{Plots.GRBackend}, Plots.Subplot{Plots.GRBackend}, Plots.Subplot{Plots.GRBackend})
Stacktrace:
[1] error(s::String)
@ Base ./error.jl:35
[2] macro expansion
@ ~/.julia/packages/RecipesPipeline/BGM3l/src/user_recipe.jl:158 [inlined]
[3] apply_recipe(plotattributes::AbstractDict{Symbol, Any}, v1::Any, v2::Any, v3::Any, v4::Any, vrest::Any)
@ RecipesPipeline ~/.julia/packages/RecipesBase/BRe07/src/RecipesBase.jl:300
[4] _process_userrecipes!(plt::Any, plotattributes::Any, args::Any)
@ RecipesPipeline ~/.julia/packages/RecipesPipeline/BGM3l/src/user_recipe.jl:38
[5] recipe_pipeline!(plt::Any, plotattributes::Any, args::Any)
@ RecipesPipeline ~/.julia/packages/RecipesPipeline/BGM3l/src/RecipesPipeline.jl:72
[6] _plot!(plt::Plots.Plot, plotattributes::Any, args::Any)
@ Plots ~/.julia/packages/Plots/du2dt/src/plot.jl:223
[7] plot!(::Plots.Plot, ::Any, ::Vararg{Any}; kw...)
@ Plots ~/.julia/packages/Plots/du2dt/src/plot.jl:213
[8] plot(::Plots.Plot{Plots.GRBackend}, ::Plots.PlaceHolder, ::Vararg{Any}; kw::@Kwargs{layout::Tuple{Int64, Int64}, legend::Bool})
@ Plots ~/.julia/packages/Plots/du2dt/src/plot.jl:206
[9] plot(::Plots.Subplot, ::Any, ::Vararg{Any}; kw...)
@ Plots ~/.julia/packages/Plots/du2dt/src/plot.jl:293
[10] top-level scope
@ ~/UNI/SLT/mini_project_2020/prova.jl:18
[11] include(fname::String)
@ Base.MainInclude ./client.jl:489
[12] top-level scope
@ REPL[7]:1
in expression starting at /home/ape/UNI/SLT/mini_project_2020/prova.jl:18
```

What’s the problem?

`Julia Version 1.10.4 (2024-06-04)`

`Plots Version 1.40.5`