CODEWITHSUNDEEP
rgraphplotregression
Barry
Barry

Reputation: 397

Polynomial regression (second order) plot in R

I am doing a polynomial regression in R for the following data but I cannot display the correct graph of the polynomial of 2rd degree. I got the equation of polynomial of degree 2 right, however I did something wrong in the last part of the script. Could anyone help? Thanks

Here is my script:

Vegetation_Cover <- c(5,0,10,40,100,30,80,2,70,2,0)
NDVI <- c(0.35,0.32,0.36,0.68,0.75,0.48,0.75,0.35,0.70,0.34,0.28)

plot(Vegetation_Cover,NDVI, main=list
("Vegetation Cover and NDVI",cex=1.5),pch=20,cex=1.4,col="gray0")

sample1 <- data.frame(Vegetation_Cover, NDVI)
sample1

fit2 <- lm(sample1$NDVI ~ sample1$Vegetation_Cover + I(sample1$Vegetation_Cover^2))

summary(fit2)
lm(formula = sample1$NDVI ~ sample1$Vegetation_Cover + I(sample1$Vegetation_Cover^2))
anova(fit2)

pol2 <- function(x) fit2$coefficient[3]*x^2 + fit2$coefficient[2]*x + fit2$coefficient[1]
plot(sample1$Vegetation_Cover, sample1$NDVI, type="p", lwd=3)
pol2 <- function(x) fit2$coefficient[3]*x^2 + fit2$coefficient[2]*x + fit2$coefficient[1]
curve(pol2, col="red", lwd=2)
points(sample1$Vegetation_Cover, sample1$NDVI, type="p", lwd=3)

Upvotes: 2

Views: 4506

Answers (1)

josliber
josliber

Reputation: 44299

It looks like you're just missing add=TRUE for your call to curve. This seems to plot what you're looking for:

pol2 <- function(x) fit2$coefficient[3]*x^2 + fit2$coefficient[2]*x + fit2$coefficient[1]
plot(sample1$Vegetation_Cover, sample1$NDVI, type="p", lwd=3)
curve(pol2, col="red", lwd=2, add=T)

Upvotes: 2

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