title: Add units to Y axes
body: <p>I'm using <code>plotrix.twoord</code> to compare two plots in a single graph using this code:</p>
  
  <pre><code>library(plotrix)
  
  mempool&lt;-read.csv("mempool-size.csv")
  mempool$date &lt;- as.Date(mempool$date)
  fees&lt;-read.csv("transaction-fees.csv")
  fees$date &lt;- as.Date(fees$date)
  
  twoord.plot(mempool$date,mempool$value,fees$date,fees$value,type="b",
              ylab="Unconfirmed transactions (B)",rylab="Fees paid per day (BTC)",
              main="Fees and mempool size since 2017",
              )
  </code></pre>
  
  <p>This prints a graph like this:</p>
  
  <p><a href="https://i.stack.imgur.com/8xuK3.png" rel="nofollow noreferrer">Plot</a></p>
  
  <p>This looks almost right but the left axis looks a bit odd. I would like it to show the unit (B) after the values - preferably with metric prefixes instead of scientific notation (20MB instead of 2e07B).</p>
  
  <p>I'm not sure if plotrix.twoord provides an option like this, so I tried to use ggplot instead:</p>
  
  <pre><code>ggplot() + 
    geom_line(data=fees, aes(x=date, y=value), color='black') + 
    geom_line(data=mempool, aes(x=date, y=value), color='red')
  </code></pre>
  
  <p>But now the fees line is just flat because it doesn't have it's own axis.</p>
  
  <p>You can find the data here if you want to try it yourself:
  <a href="http://www.sharecsv.com/s/07076796191d9e414e5b50840d9ed7ad/mempool-size.csv" rel="nofollow noreferrer">http://www.sharecsv.com/s/07076796191d9e414e5b50840d9ed7ad/mempool-size.csv</a>
  <a href="http://www.sharecsv.com/s/72a183dd3ab0df781eacbfee81999548/transaction-fees.csv" rel="nofollow noreferrer">http://www.sharecsv.com/s/72a183dd3ab0df781eacbfee81999548/transaction-fees.csv</a></p>