title: Unexpected quantile relationship
body: <p>I want to fit a quantile regression model to my observed data, which clearly show a triangular relationship between the response and predictor variables:
  <img src="https://i.stack.imgur.com/VmjIf.png" alt="enter image description here"></p>
  
  <p>When I do:</p>
  
  <pre><code>library("quantreg")
  m1 &lt;- rq(Y~ X, tau = 0.75, data=mydata)
  
  summary(m1)
  
  Call: rq(formula = Y ~ X, tau = 0.75, data = mydata)
  
  tau: [1] 0.75
  
  Coefficients:
  
              coefficients lower bd upper bd
  (Intercept) 3.42758      1.80850  4.74463 
  
  X           0.27879      0.07132  0.82591 
  </code></pre>
  
  <p>It founds a positive relationship (in red), when it should be negative looking at the points in the graph, right? Maybe I'm missing something but it looks like a wrongh tau value. I tried with t=0.97 and t=0.90 (in gray), but the same pattern is produced.</p>
  
  <p>Then, when I do:</p>
  
  <pre><code>m1.all &lt;- rq(Y~ X, tau = seq(0.05, 0.95, by = 0.05), data=mydata)
  
  m1.plot &lt;- summary(m1.all)
  </code></pre>
  
  <blockquote>
    <p>Warning messages:<br>
        1: In rq.fit.br(x, y, tau = tau, ci = TRUE, ...) :<br>
          Solution may be nonunique<br>
        2: In rq.fit.br(x, y, tau = tau, ci = TRUE, ...) :<br>
          Solution may be nonunique<br>
        3: In rq.fit.br(x, y, tau = tau, ci = TRUE, ...) :<br>
          Solution may be nonunique<br>
        4: In rq.fit.br(x, y, tau = tau, ci = TRUE, ...) :<br>
          Solution may be nonunique</p>
  </blockquote>
  
  <pre><code>plot(m1.plot)
  </code></pre>
  
  <blockquote>
    <p>Error in plot.window(...) : infinite axis extents [GEPretty(-inf,inf,5)]</p>
  </blockquote>
  
  <p>I only obtain the plot for the intercept, but not for the coefficients.</p>
  
  <p>What I'm doing wrong?</p>
  
  <p>I provide <a href="https://www.dropbox.com/s/2qd2ykh1ttt9mg5/mydata.csv?dl=0" rel="nofollow noreferrer">here</a> mydata. I expect a negative relationship similar to results shown by Cade &amp; Noon 2003 in Fig. 1 (see <a href="http://www.econ.uiuc.edu/~roger/research/rq/QReco.pdf" rel="nofollow noreferrer">here</a>).</p>