Choosing variables to plot from DifferentialEquations solution

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
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())

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