streamplot

using Makie

 struct FitzhughNagumo{T}
     ϵ::T
     s::T
     γ::T
     β::T
 end

 P = FitzhughNagumo(0.1, 0.0, 1.5, 0.8)
 f(x, P::FitzhughNagumo) = Point2f0(
     (x[1]-x[2]-x[1]^3+P.s)/P.ϵ,
     P.γ*x[1]-x[2] + P.β
 )
 f(x) = f(x, P)
 streamplot(f, -1.5..1.5, -1.5..1.5, colormap = :magma)