title: colorRamp returns 0
body: <p>I'm trying to plot lines and color the lines based on the probability of that connection.  Given a vector of probabilities, I use:</p>
  
  <pre><code>colfunc &lt;- colorRamp(c("white", "red"))
  colors &lt;- colfunc(probs)
  </code></pre>
  
  <p>colors is then an nx3 matrix of rgb values.  However, colfunc quite often returns a 0 value, so when i attempt to plot using these colors, R complains</p>
  
  <pre><code>Error in col2rgb(colors) : numerical color values must be positive
  </code></pre>
  
  <p>Is there an error in the way I am defining my color function?</p>