title: Add a function to ggplot
body: <br>
  I would like to add this function 
  <pre><code>ma &lt;- function(x,n=11){filter(x,rep(1/n,n),method = &quot;convolution&quot;, sides=2)}
  </code></pre>
  <p>` <br>
  to my data: <br></p>
  <pre><code>ggplot(data, aes(y=volume, x=time)) +
     geom_line(color= &quot;black&quot;, size=0.2, alpha=0.9) +
     theme_classic() +
     ggtitle(&quot; Volume&quot;)
  </code></pre>