title: unable to set xlim and ylim using min() and max() in ggplot
body: <p>I am missing something crucial here and can't see it.</p>
  
  <p>Why does min and max not work to set the axis limits?</p>
  
  <pre><code>mtcars %&gt;%  
    select(mpg, cyl, disp, wt) %&gt;% 
    filter(complete.cases(disp)) %&gt;% 
    ggplot() +
    geom_point(aes(x=mpg, y=disp, colour=cyl), size=3) +
    xlim(min(mpg, na.rm=TRUE),max(mpg, na.rm=TRUE)) +
    ylim(min(disp, na.rm=TRUE),max(disp, na.rm=TRUE)) +
    scale_colour_gradient(low="red",high="green", name = "cyl")
  </code></pre>
  
  <p>This works:</p>
  
  <pre><code>    mtcars %&gt;%  
        select(mpg, cyl, disp, wt) %&gt;% 
        filter(complete.cases(disp)) %&gt;% 
        ggplot() +
        geom_point(aes(x=mpg, y=disp, colour=cyl), size=3) +
  #      xlim(min(mpg, na.rm=TRUE),max(mpg, na.rm=TRUE)) +
  #      ylim(min(disp, na.rm=TRUE),max(disp, na.rm=TRUE)) +
        scale_colour_gradient(low="red",high="green", name = "cyl")
  </code></pre>