Dynamic height and width for knitr plots

You can for example define the width in another chunk, then use it

```{r,echo=FALSE}
x <- data.frame(x=factor(letters[1:3]),y=rnorm(3))
len = length(unique(x$x))
```


```{r fig.width=len, fig.height=6}

plot(cars)
```

I just found a great blog post about this.

Read more at Michael J Williams' blog - I've shamelessly pilfered the code, so more detail there. Remember to set chunk options to results = "asis".

Say you want to output a bunch of plots using a loop but you want them to each have different sizes. Define the following function (again, I'm just copy-pasting here):

subchunkify <- function(g, fig_height=7, fig_width=5) {
  g_deparsed <- paste0(deparse(
    function() {g}
  ), collapse = '')

  sub_chunk <- paste0("
  `","``{r sub_chunk_", floor(runif(1) * 10000), ", fig.height=",
   fig_height, ", fig.width=", fig_width, ", echo=FALSE}",
  "\n(", 
    g_deparsed
    , ")()",
  "\n`","``
  ")

  cat(knitr::knit(text = knitr::knit_expand(text = sub_chunk), quiet = TRUE))
}

And use the function like this, defining your own figure sizes:

```{r echo=FALSE, results='asis'}
g <- ggplot(economics, aes(date, unemploy)) + 
  geom_line()
subchunkify(g, 10, 3)
subchunkify(g, 7, 7)
```

Or let the data define the sizes:

```{r echo=FALSE, results='asis'}
g <- ggplot(economics, aes(date, unemploy)) + 
  geom_line()
for (i in seq(2, 5)) {
  subchunkify(g, i / 2, i)
}
```

In the post Michael warns that you must be careful:

Since we’re using results='asis', if we want to output text or headings or anything else from the chunk, we must use raw HTML, not Markdown, and we must use cat() to do this, not print(). For instance:

g <- ggplot(economics, aes(date, unemploy)) + 
  geom_line()

cat('<h2>A Small Square Plot</h2>')
subchunkify(g, 3, 3)

Again, not my work... head on over to that lovely blog post for more details. Hope this works for you.