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.

Answer 2

You should add argument bg="red" or bg="blue" inside plot() to get the filled symbols with particular color.