How do I obtain values of a curve f(x1,x2) = 0 in R to visualize it? <p>I have <code>ggplot</code> scatters of points and I'm trying to visualize the different decision boundaries i get with <code>glm</code>. I can grab the coefficients from <code>glm</code> and I end up with something like <code>f(x1,x2) = 0</code> which could have power interactions so I can't just try <code>abline</code>. What's the simplest way to visualize it on the scatter? I was thinking if there's a "solver" maybe which will return a frame of points on a grid that I can then <code>geom_line</code> maybe?</p>