plot functions with filled point symbols and legend

Question

I want to plot two functions in different colors and point styles with a corresponding legend, in plain R.

I have several questions:

1. I

Answer 1

I think maybe you'd have better luck using curve:

curve(x^(2) / 2,from = 0,to = 100,col = 'red',type = 'p',pch = 16,n = 20)
curve((1-x^(2))/2 + 5000,from = 0,to = 100,col = 'blue',type = 'p',pch = 15,add = TRUE,n = 20)
legend("topleft", 
        inset = c(0,0.4), 
        cex = 1.5, 
        bty = "n", 
        legend = c("A", "B"), 
        text.col = c("red", "blue"),
        col = c("red", "blue"), 
        pch = c(16,15))

Note that I had to tweak your functions slightly, to get output that matched your image.

To avoid specifying color and fill separately (which in general is how things are done in R) I used some older "legacy" symbols. Using curve is often much simpler for plotting functions or expressions. It also gives you a more convenient way to specify the grid of points to evaluate on. It also has an add argument that allows you to skip the awkward par hacking you engaged in.