How to properly plot elliptic curves using CeTZ?

Questions
1

Hi, I am trying to plot some elliptic curves with CeTZ, here is my code so far:

#import "@preview/cetz:0.3.1"
#import "@preview/cetz-plot:0.1.0": plot, chart

#let EC_func(x) = {
  let y = x*x*x - 4*x + 6
  if y < 0 {
    return 0
  }
  return calc.sqrt(y)
}


#let EC_drawing = cetz.canvas({
  plot.plot(
  size: (10, 10), 
  x-tick-step: 5, 
  y-tick-step: 20, 
  x-min: -5, 
  max: 10, 
  x-grid: true,
  y-grid: true,
  
  {
    plot.add(domain: (-5, 10), EC_func)
    plot.add(domain: (-5, 10), t => (t, -EC_func(t)))
  })
})


#EC_drawing

Screenshot 2024-10-30 200922

Obviously, it’s not perfect. There are 2 things I haven’t figured out how to do:

  • when I plot the function, there are some x-values not in the domain (which now get set to 0, see the “beak” on the left). Is there a way to ignore these points? In the example above, these would be points <= 2.5 so I could just set domain: (-2.5, 10), but this way gets annoying quick if I want to plot multiple curves, doubly so if they are disconnected in the middle
  • Less importantly, is there an easy way to plot both the positive and negative side? Of course I can do it like I did and manually set the color for both, but if there’s an easier way that’d be great

Thanks!

2

This is maybe not you’re asking for or what you want to hear… but using cetz is simply not the “correct” way (for now) to plot elliptic curves. It inherently doesn’t support implicit plots (where you give it an equation and it draws all solutions), but only graphs of functions.

I think you should use a library which is precisely built for all sorts of scientific plotting, like e.g. matplotlib together with other libraries, and then use the exported image in Typst.

In your case, sympy supports implicit plotting: Plotting - SymPy 1.13.3 documentation

3

okay, thanks. I’ll look into matplotlib :slight_smile:

5

Hey @aarnent, welcome to the forum! I’ve updated your post title to better fit our guidelines for question posts: How to post in the Questions category

Make sure your post title is a question you’d ask to a friend about Typst. :wink: