title: How to change labels (legends) in ggplot?
body: <p>My code is like below, I want to change the label of the ggplot, but R always remind me:</p>
  
  <pre><code>Error in unit(tic_pos.c, "mm") : 'x' and 'units' must have length &gt; 0
  </code></pre>
  
  <p>What should I do?</p>
  
  <pre><code>ggplot(mat,aes(x=sales,col=type))+
    geom_density()+labels("red_sold","blue_sold","yellow_sold")
  </code></pre>