Quadratic spline

Question

Is it there a way to adjust a quadratic spline (instead of a cubic one) to some data?

I have this data and I don't seem to find the appropiate function in R to do this.

Answer 1

Expanding just a bit on the comments above, you can use a B-spline basis (implemented in function splines::bs()), setting degree=2 rather than the default degree=3:

library(splines)

## Some example data
set.seed(1)
x <- 1:10
y <- rnorm(10)

## Fit a couple of quadratic splines with different degrees of freedom
f1 <- lm(y ~ bs(x, degree = 2))  # Defaults to 2 - 1 = 1 degree of freedom
f9 <- lm(y ~ bs(x, degree = 2, df=9))

## Plot the splines
x0 <- seq(1, 10, by = 0.1)
plot(x, y, pch = 16)
lines(x0, predict(f1, data.frame(x = x0)), col = "blue")
lines(x0, predict(f9, data.frame(x = x0)), col = "red")