# Specify the admissible region of SIS model when plotting its phase portrait, using streamplot

**URL:** https://discourse.julialang.org/t/specify-the-admissible-region-of-sis-model-when-plotting-its-phase-portrait-using-streamplot/67688
**Category:** Modelling & Simulations
**Tags:** question
**Created:** [September 4, 2021, 11:48am UTC](https://discourse.julialang.org/t/specify-the-admissible-region-of-sis-model-when-plotting-its-phase-portrait-using-streamplot/67688 "2021-09-04T11:48:59Z")
**Posts on this page:** 1
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### Author: ![GoH](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goh/32/28770_2.png) [@GoH](https://discourse.julialang.org/u/GoH)
#### Post date: [September 4, 2021, 1:15pm UTC](https://discourse.julialang.org/t/specify-the-admissible-region-of-sis-model-when-plotting-its-phase-portrait-using-streamplot/67688/3 "2021-09-04T13:15:26Z")

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> What do you mean precisely with that? Physically, the model conserves S+I=1, so yes, actually evolving the model with initial conditions that satisfy S+I = 1 would keep it on this manifold. But that’s not what `streamplot` does. it just plots the vector field.

I think the trajectories supposed to strictly on the line of S+I=1, and the phase portrait of SIS model is like this:  
 ![image](https://global.discourse-cdn.com/julialang/original/3X/2/f/2fd4f1c4337c91602a9bbd12accfb9b797f7529c.png)

> You can’t use `streamplot` for what you want.

I initially found the solution in the post [Plotting a phase portrait of a differential equation](https://discourse.julialang.org/t/plotting-a-phase-portrait-of-a-differential-equation/29208) is streamplot, so I use it. am I misunderstanding?

Also, I am not sure whether the following understanding is correct:

**Plotting the vector field** : plot the vector on each point, its direction is obtained according to the differential equations (\frac{dS}{dt},\frac{dI}{dt},\frac{dR}{dt}). So, we can use `streamplot`.

**Plotting the phase space**  
plot trajectories starting from every possible initial  
state (initial condition). And, your suggestion is `quiver` plot?

Thanks!

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