`geom_smooth` with variable degree polynomial in the formula

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I have the following ggplot2 code that plots multiple polynomial fits with various degrees:

library(ggplot2)

set.seed(1234)
n = 400
x = rnorm(n, sd=0.4)
y = -x + 2*x^2 - 3*x^3 + rnorm(n,sd=0.75)
df = data.frame(x=x,y=y)

deg = c(1,2,3,10)
cols = c("red","green","blue","orange")
ggplot(df, aes(x=x,y=y)) + 
  geom_point() + 
  geom_smooth(method = "lm", formula= y~poly(x,deg[1]), se=F, col=cols[1]) +
  geom_smooth(method = "lm", formula= y~poly(x,deg[2]), se=F, col=cols[2]) +
  geom_smooth(method = "lm", formula= y~poly(x,deg[3]), se=F, col=cols[3]) +
  geom_smooth(method = "lm", formula= y~poly(x,deg[4]), se=F, col=cols[4])

I would like to avoid repeating the geom_smooth line for every degree. But I can't figure out how to get geom_smooth to understand a dynamic degree passed through a variable. Is there a more elegant solution to the above? It would be nice to also automatically change the colors without the need to explicitly pass the cols vector. (Default color scheme is fine).

I have tried to use as.formula(paste0("y~poly(x,",deg[i],")")) through a loop without much luck (and loops don't seem to be the right approach with ggplot.)

ggplot2 r
1 Answers

You can add a list of plot elements to a ggplot, so you could use map to create a list of four geom_smooth calls, one for each degree in deg.

library(tidyverse)

ggplot(df, aes(x=x,y=y)) + 
  geom_point() +
  map(1:length(deg), 
      ~geom_smooth(method="lm", formula=y~poly(x, deg[.x]), se=F, col=cols[.x]))

You can also add a legend if you wish. For example:

ggplot(df, aes(x=x,y=y)) + 
  geom_point(colour="grey60") +
  map(1:length(deg), 
      ~geom_smooth(method="lm", formula=y~poly(x, deg[.x]), se=F,
                   aes(color=factor(deg[.x])))) + 
  scale_colour_manual(name="Degree", breaks=deg, values=set_names(cols, deg)) +
  theme_bw() +
  theme(legend.text.align=1)

enter image description here

If you're happy with the default colours, change the scale_colour_manual line to:

scale_colour_discrete(name="Degree", breaks=deg) +
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