fit1 <- lm(y ~ x, data = data)


fit2 <- lm(y ~ x + I(x^2), data = data)


plot(data$x, data$y, xlab = "x", ylab = "y")
abline(fit1)

lines(data$x, fitted(fit2), lty = 1, col = "red")

画出的图是好多线，而不是一根曲线