How to change the appearance of the best fit line in R-CodePudding

I am trying to add a polynomial best fit line but for some reason, it appears to be drawn back and forth on the plot. I was wondering what I should do to fix the line

yhat_pol = lm(age~poly(height,2), data = Loblolly)
yhat_lin = lm(age~height, data = Loblolly)

plot(x=Loblolly$height, y=Loblolly$age, pch=16, xlab = "Height (ft)", ylab = "Age (yr)", main = "Height vs Age of Loblloy Pine Trees")
lines(Loblolly$height, predict(yhat_pol), type="l", col="green", lwd=2)
lines(Loblolly$height, predict(yhat_lin), type="l", col="red", lwd=2)
lines(sort(Loblolly$height), fitted(yhat_lin)[order(Loblolly$height)])
legend("topleft", 
        legend = c("linear","polynomial"), 
        col = c("red","green"),
        lty = 1, lwd=3)

	height	age	Seed
1	4.51	3	301
15	10.89	5	301
29	28.72	10	301
43	41.74	15	301
57	52.70	20	301
71	60.92	25	301
2	4.55	3	303
16	10.92	5	303
30	29.07	10	303
44	42.83	15	303

CodePudding user response：

You almost had it.

lines(sort(Loblolly$height), predict(yhat_pol)[order(Loblolly$height)], type="l", col="green", lwd=2)

I don't think the order of the data causes these problems if you use ggplot2.

CodePudding user response：

You didn't ask for it, but here is - as pointed out by Brian Montgomery - a solution using ggplot2:

library(dplyr)
library(tidyr)
library(ggplot2)


Loblolly %>%
  mutate(age_pred_pol = predict(yhat_pol),
         age_pred_lin = predict(yhat_lin)) %>% 
  ggplot(aes(x = height, y = age))  
  geom_point()  
  geom_line(aes(y = age_pred_pol, color = "Polynomial Best Fit"))  
  geom_line(aes(y = age_pred_lin, color = "Linear Best Fit"))  
  labs(x = "Height (ft)", y = "Age (yr)", title = "Height vs Age of Loblloy Pine Trees")  
  scale_colour_manual(name = "Prediction",
                      breaks = c("Polynomial Best Fit", "Linear Best Fit"),
                      values = c("red", "green"))  
  theme_bw()

This returns