title: ggplot2 axis transformation by constant factor
body: <p>In a ggplot2 density plot (geom_density) I have the following y-axis labels</p>
  
  <ul>
  <li>0.000</li>
  <li>0.005</li>
  <li>0.010</li>
  <li>0.015</li>
  <li>0.020</li>
  </ul>
  
  <p>What is the correct way to change them to something like</p>
  
  <ul>
  <li>0</li>
  <li>5</li>
  <li>10</li>
  <li>15</li>
  <li>20</li>
  </ul>
  
  <p>possibly with the automatic adding of "10^3 x density" to the label.  In the past I've just multiplied my data and manually changed the label, but in this case the y-axis data is generated for me by the density plot.</p>
  
  <p>I'm aware that I can write things like <code>scale_y_continuous(trans="log10")</code>, but have not found any way to do a simple multiplicative constant, or define a custom transform.</p>