Messy plot when plotting predictions of a polynomial regression using lm() in R

Question

I am building a quadratic model with lm in R:

y <- data[[1]]
x <- data[[2]]
x2 <- x^2

quadratic.model = lm(y ~ x + x2)

Now I wan

Answer 1

You need order():

P <- predict(quadratic.model)
plot(y~x)
reorder <- order(x)
lines(x[reorder], P[reorder])

My answer here is related: Problems displaying LOESS regression line and confidence interval