title: R data analysis - qqplot - Error msg: unimplemented type 'list' in 'greater'
body: <p>I am facing an error message when calling function <code>qqplot</code> in car package. Here is details:</p>
  
  <p>I use <code>lm</code> to do linear analysis for a data-set. And I want to check this model if it is good enough. Then I use <code>qqplot</code> in car package. Unfortunately, it returns an error message saying: </p>
  
  <blockquote>
    <p>Error in rank(x, ties.method = "min", na.last = "keep") :unimplemented
    type 'list' in 'greater'.</p>
  </blockquote>
  
  <p>Could someone help me on this ? Cause I followed the steps in the book named "R in Action", but it didn't return the expected result.</p>
  
  <p>Code:</p>
  
  <pre><code>library(car)
  states &lt;- as.data.frame(state.x77[,c('Murder','Population','Illiteracy',
                                  'Income','Frost')])
  mur &lt;- lm(Murder~Population+Illiteracy+Income+Frost,states)
  qqplot(mur)
  </code></pre>