Ability to plot parameters

[mtiller-jh]

Is your feature request related to a problem? Please describe.

It is quite common, for example in control systems, to want to compare a trajectory with a set point. For some cases, that set point may be a parameter. However, currently trying to plot a parameter leads to an ArgumentError: Collection must contain exactly 1 element error.

Describe the solution you’d like

I would like parameters to be plotable just like any other symbol.

Describe alternatives you’ve considered

The only alternative I can imagine is not plotting them. In such a case, an error explicitly saying "plotting of parameters is not supported" would certainly be preferable to the error above. But I suspect providing a good error message is about as much effort as actually plotting the variable.

[oscardssmith]

Do you have a MWE of this issue?

[ahharisankar]

This dyad code

`component Plot_parameter
parameter a::Real =10
parameter b::Real =10
variable s::Real

relations
s=(a+b)*time
end`

generates this Julia code :

`
@component function Plot_parameter(; name, a=10, b=10)

__params = Any[]
append!(__params, @parameters (a::Float64 = a))
append!(__params, @parameters (b::Float64 = b))

__vars = Any[]
append!(__vars, @variables (s(t)))

__constants = Any[]

__systems = ODESystem[]

__defaults = Dict()

__initialization_eqs = []

__eqs = Equation[]
push!(__eqs, s ~ (a + b) * t)

return ODESystem(__eqs, t, __vars, __params; systems=__systems, defaults=__defaults, name, initialization_eqs=__initialization_eqs)
end
export Plot_parameter
`

and on plotting we get this error :

julia> plot(sol,idxs=[sys.a]) ERROR: ArgumentError: Collection must contain exactly 1 element Stacktrace: [1] _only @ .\iterators.jl:1561 [inlined] [2] only(s::Set{Any}) @ Base .\set.jl:178 [3] macro expansion @ C:\Users\ahhar\.julia\packages\SciMLBase\Ha7rZ\src\solutions\solution_interface.jl:253 [inlined] [4] apply_recipe(plotattributes::AbstractDict{Symbol, Any}, sol::SciMLBase.AbstractTimeseriesSolution) @ SciMLBase C:\Users\ahhar\.julia\packages\RecipesBase\BRe07\src\RecipesBase.jl:300 [5] _process_userrecipes!(plt::Any, plotattributes::Any, args::Any) @ RecipesPipeline C:\Users\ahhar\.julia\packages\RecipesPipeline\BGM3l\src\user_recipe.jl:38 [6] recipe_pipeline!(plt::Any, plotattributes::Any, args::Any) @ RecipesPipeline C:\Users\ahhar\.julia\packages\RecipesPipeline\BGM3l\src\RecipesPipeline.jl:72 [7] _plot!(plt::Plots.Plot, plotattributes::Any, args::Any) @ Plots C:\Users\ahhar\.julia\packages\Plots\Ec1L1\src\plot.jl:223 [8] plot(args::Any; kw...) @ Plots C:\Users\ahhar\.julia\packages\Plots\Ec1L1\src\plot.jl:102 [9] top-level scope @ REPL[10]:1