Using a plot call in a function without evaluating it - r

Question

In a Rmarkdown document, I want to accellerate the knitting process by only building the plots when they have not been already built and saved.
I did this using the code below, with a simple example.
x = rnorm(10)
if (! "figurename" %in% dir("figure")) {
png("figure/figurename.png")
hist(x)
dev.off()
}
Now I want to make a function, that does the above command automatically, with a plot call as input. Also, the plot call should not have been evaluated (too slow!). I learned about the substitute command and I wrote this :
x = rnorm(10)
plot_call = substitute(hist(x))
function(plot_call, figurename){
if (! figurename %in% dir("figure")) {
png(file.path("figure", figurename))
eval(plot_call)
dev.off()
}
knitr::include_graphics(file.path("figure", figurename))
}
I have two issues with this :
it does not seem to work with multiple lines plot calls
it seems like a dubious hack
What do you think? Is there a better way ?

Answer 1

A more formal way to cache code blocks is to leverage chunk options. Adding a simple cache = TRUE to the block header will force your plot to be re-evaluated each time block options or the code itself will change:
```{r expensive_plot, cache = TRUE}
# Some expensive plot
df %>%
ggplot(aes(x, y)) +
geom_point()
```
If the plot needs to be recomputed each time there's a change in the underlying data, it's possible to invalidate cache each time the file 'last edited' field changes by adding cache.extra = file.mtime('your-csv.csv') to your options.