title: Set ylim with stacked area plot
body: <p>I am trying to do a stacked area plot in order to compare outcomes under three scenarios. </p>
  
  <pre><code>ggplot(FIREP_All,aes(x = Time, y = Houses, fill=Risk_Class, group=  Risk_Class))+ geom_line(aes(ymax=Houses), position="stack", colour='darkgrey')+ geom_area(alpha=.75) +
    scale_fill_brewer(type="seq", palette=18,name = "Fire Risk Class")+facet_grid(~Scenario)  
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/uIFx5.png" alt="enter image description here"></p>
  
  <p>Although I know that stacked area plots are cumulative, I want to set a ylim, so that it is easier to see difference between the scenarios. Preferably ylim(1e7,2.5e7).  The problem is that it interferes with the calculations.</p>
  
  <pre><code>    ggplot(FIREP_All,aes(x = Time, y = Houses, fill=Risk_Class, group=  Risk_Class))+ geom_line(aes(ymax=Houses), position="stack", colour='darkgrey')+ geom_area(alpha=.75) + scale_fill_brewer(type="seq", palette=18,name = "Fire Risk Class")+facet_grid(~Scenario)  
  
  +ylim(1e7,2.5e7)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/l7vAd.png" alt="enter image description here"></p>
  
  <p>Any thoughts on a work around? </p>