Adding a 3rd order polynomial and its equation to a ggplot in r

Question

I have plotted the following data and added a loess smoother. I would like to add a 3rd order polynomial and its equation (incl. the residual) to the plot. Any advice?

Answer 1

Part 1: to fit a polynomial, use the arguments:

• method=lm - you did this correctly
• formula=y ~ poly(x, 3, raw=TRUE) - i.e. don't wrap this in a call to lm

The code:

p + stat_smooth(method="lm", se=TRUE, fill=NA,
                formula=y ~ poly(x, 3, raw=TRUE),colour="red")

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Part 2: To add the equation:

• Modify your functionlm_eqn() to correctly specify the data source to lm - you had a closing parentheses in the wrong place
• Use annotate() to position the label, rather than geom_text

The code:

lm_eqn = function(df){
  m=lm(y ~ poly(x, 3), df)#3rd degree polynomial
  eq <- substitute(italic(y) == a + b %.% italic(x)*","~~italic(r)^2~"="~r2,
                   list(a = format(coef(m)[1], digits = 2),
                        b = format(coef(m)[2], digits = 2),
                        r2 = format(summary(m)$r.squared, digits = 3)))
  as.character(as.expression(eq))
}


p + annotate("text", x=0.5, y=15000, label=lm_eqn(df), hjust=0, size=8, 
             family="Times", face="italic", parse=TRUE)