title: Error: could not find function "unit"
body: <p>While trying to modify theme settings this simple code gives the following error:</p>
  
  <pre><code>library(ggplot2)
  theme_nogrid &lt;- theme_set(theme_update(  
                    plot.margin=unit(c(.25, .25, .25, .25), "in"),))
  
  Error in do.call(theme, list(...)) : could not find function "unit"
  </code></pre>
  
  <p><code>R</code> gives me this error for any element that uses 'unit'. Any other settings that do not call 'unit' work fine. I am running R v.2.15.2 (64-bit Windows). </p>
  
  <p>I extensively searched online about this problem and found nothing. 
  I appreciate any suggestions to the problem.</p>