How to make PDF output and code output look right in Rmarkdown

Question

I'm working with a large collection of R Markdown files that are collectively used to create a book (PDF output) using bookdown. For long lines of code (that will show up in the book), I've been writing it out close to column 80, at which point I insert a hard line break, tab (4 spaces) over, and continue the code. This is what I think looks good in the text (for readers). However, I'm running into problems when this is (for example) part of a ggplot() function. Here's a quick (meaningless) example:

testx <- seq(1:100)
testy <- rnorm(testx)
testdat <- as.data.frame(cbind(testx, testy))
g <- ggplot(testdat, aes(x = testx/100, y = testy))
g <- g + geom_point() + geom_line()
g <- g + labs(title = "The Posterior Distribution",
    caption = "The mode and 95% HPD intervals are the dot and horizontal line at 
    the bottom, respectively.",
    x = expression(beta[1]~(slope)))
g

In this example, I need to break the caption() argument across two lines because it is long, but I do NOT want it to break in the actual caption.

Is there a way to do this?