fixup! refactor: add new *_symbols methods, corresponding singletons,…

… update tests

docs/Project.toml

@@ -2,6 +2,7 @@
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 ModelingToolkit = "961ee093-0014-501f-94e3-6117800e7a78"
 OrdinaryDiffEq = "1dea7af3-3e70-54e6-95c3-0bf5283fa5ed"
+Plots = "91a5bcdd-55d7-5caf-9e0b-520d859cae80"
 SymbolicIndexingInterface = "2efcf032-c050-4f8e-a9bb-153293bab1f5"
 
 [compat]

docs/src/usage.md

@@ -4,7 +4,7 @@ This tutorial will cover ways to use the interface for types that implement it.
 Consider the following example:
 
 ```@example Usage
-using ModelingToolkit, OrdinaryDiffEq, SymbolicIndexingInterface
+using ModelingToolkit, OrdinaryDiffEq, SymbolicIndexingInterface, Plots
 
 @parameters σ ρ β
 @variables t x(t) y(t) z(t) w(t)
@@ -46,7 +46,7 @@ sol[x]
 ```
 
 This also works for arrays or tuples of variables, including observed quantities and
-independent variables, and for interpolating solutions:
+independent variables, for interpolating solutions, and plotting:
 ```@example Usage
 sol[[x, y]]
 ```
@@ -60,13 +60,17 @@ sol(1.3, idxs=x)
 ```
 
 ```@example Usage
-sol(1.3, idxs=[x, w]) # cannot use tuples of variables for interpolation
+sol(1.3, idxs=[x, w])
 ```
 
 ```@example Usage
 sol(1.3, idxs=[:y, :z])
 ```
 
+```@example Usage
+plot(sol, idxs=x)
+```
+
 If necessary, `Symbol`s can be used to refer to variables. This is only valid for
 symbolic variables for which [`hasname`](@ref) returns `true`. The `Symbol` used must
 match the one returned by [`getname`](@ref) for the variable.