How to adjust the density of streamplot in Julia?

Hi, I am trying to generate the phase space of the SI model, based on the example code in https://lazarusa.github.io/BeautifulMakie/FlowFields/streamplots3/

my code (in Pluto notebook) is

begin
	using CairoMakie
	let
		β=0.4
		μ=0.1
	    PP_SIS(S,I) = Point(-β*S*I+μ*I,-β*S*I-μ*I)
	    
	    fig = Figure(resolution = (700, 400), fontsize = 20)
	    ax1 = fig[1, 1] = Axis(fig, xlabel = "S", ylabel = "I", title = "phase portrait of SIS model",
	    aspect = 1, backgroundcolor = :black)
	    
	    streamplot!(ax1, PP_SIS, 0.01..0.99, 0.01..0.99, colormap = Reverse(:plasma),
	        gridsize= (32,32), arrow_size = 8)
	    
		    fig
	end
end

and got the following result

but I found it a little bit hard to clearly observe the flows, thus I am wondering whether I can adjust the density of it…

Many thanks in advance!

do you actually have stronger flows? Why not do a heat map also. streamplotAndHeatmap