What is the best way to add 1000 regression lines to a ggplot? (Model-based bootstrapping)

Question

From a bootstrapping model I have 1000 sets of coefficients for this regression model:

y = b0 + b1x + b2(x^2)

What is the function call to plot a quadratic line if I already have the coefficients? I.E. I do not want to "fit" a linear model to my data.

I tried adding lines via a for loop to my ggplot object:

for (i in 1:1000) { reg_line <- stat_function(fun=function(x) quad$coefficients[1] + quad$coefficients[i,2]*x + quad$coefficients[i,3]*(x**2)) reg_lines <- reg_lines + reg_line}

That didn't work - it seems to only add the last line in the loop.

The reason I want to add 1000 regression lines to my plot is because it is for a homework problem - I am well aware this is not a common use case.

Sorry. Too glib. You won't be able to use geom_abline for quadratic, but are you able to use it to add 1000 straight lines on to your plot, or does it too only add the last one? To get a better response you should probalbly include a minimal reproducible example so that when we run your code we get the same result. Right now you've only given an excerpt. — Hugh
– Hugh, Commented Mar 3, 2020 at 9:53
Maybe this (the second answer): stackoverflow.com/questions/15633714/… — Edward
– Edward, Commented Mar 3, 2020 at 9:54

Edward · Accepted Answer · 2020-03-03 10:46:12Z

There may be other ways to do this, but hopefully this can give you some ideas. I used the mtcars dataset and generated some bootstrap samples for modelling. You can skip this step.

library(ggplot2) library(tidyr) library(dplyr) data(mtcars) drat=seq(min(mtcars$drat), max(mtcars$drat), length.out=100) # Bootstrap function bs <- function() { df = mtcars[sample(1:nrow(mtcars), replace=TRUE),] lm_fit <- lm(mpg ~ drat+I(drat^2), data=df) data.frame(Model=predict(lm_fit, newdata=data.frame(drat))) # Replace with your own } foo <- replicate(10, bs()) # Simulate

You would start from here since you should already have a data frame or list of predicted values from your 1,000 bootstrap models. Reshape it into a very long form to create a grouping column for the geom_line function.

foo_long <- data.frame(foo, drat) %>% pivot_longer(cols=-drat, names_to="Model", values_to="mpg") ggplot(data = mtcars, aes(x = drat, y = mpg)) + geom_point(color='blue') + geom_line(data = foo_long, aes(x=drat, y=mpg, group=Model, color=Model)) + guides(color=FALSE)

Thank you for your detailed answer. I think this is the way to do it.

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