title: Display an axis value in millions in ggplot
body: <p>I have a chart where I am charting some very large numbers, in the millions.  My audience is unlikely to understand scientific notation, so I'm hoping to label the y axis in something like "2M" for two million for example.</p>
  
  <p>Here's an example.  Showing the full value (<code>scales::comma</code>) is better than the scientific notation it defaults to, but is still a bit busy:</p>
  
  <pre><code>library(ggplot2)
  ggplot(as.data.frame(list(x = c(0, 200,100), y = c(7500000,10000000,2000000))), 
         aes(x = x, y = y)) +
    geom_point() +
    expand_limits( x = c(0,NA), y = c(0,NA)) +
    scale_y_continuous(labels = scales::comma)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/RumUd.png" rel="noreferrer"><img src="https://i.stack.imgur.com/RumUd.png" alt="enter image description here"></a></p>
  
  <p>I don't want to rescale the data, since I will be including labels with the values of the individual data points as well.</p>