I understand that vrs is used in order to choose the variables to be plotted. i.e. in this example I plotted x vs y

using DifferentialEquations, ParameterizedFunctions, Plots

attrac = @ode_def begin

dx = σy(y-x)

dy = ρ

end σ ρ

u0 = [0.0,1.0]

tspan = (0.0,10)

p = [sqrt(12π),1.0]

prob = ODEProblem(attrac, u0, tspan, p)

sol = solve(prob,alg=Trapezoid())

plot(sol,vars=(1,2))

Where one uses vars=(var1, var2) where 0, 1, 2 stand for time, x, y, respectively. Now I’d like to define a function out of these variables and plot it. For example plot z(t)=y*x