title: How to adjust line size in geom_line without obtaining another (useless) legend?
body: <p>i´d like to adjust the size of my lines (both of them), because i feel they're too skinny. 
  The following code does so, but creates a legend for size, which is useless since size has no variable that can be mapped to it.</p>
  
  <pre><code>qplot(date,value,data=graph1,geom="line",colour=variable,xlab="",ylab="",size=1)
  + scale_y_continuous(limits = c(-0.3,0.3)) + opts(aspect.ratio = 2/(1+sqrt(5))) 
  + scale_colour_manual("Variable",c(Line1="red",Line2="blue")) 
  + opts(legend.size="none")
  </code></pre>
  
  <p>My plot consists of two lines representing a time series of two different variables over the same time span. The variable is mapped to color. If I try to influence the size of the line, qplot always tries to map "size" to another parameter and display another legend. </p>
  
  <p>I also followed <a href="http://groups.google.com/group/ggplot2/browse_thread/thread/28eaca9ea56ce5cc/80faa8947fa6a01a?#80faa8947fa6a01a" rel="noreferrer">this</a> discussion, that ended with Hadley telling the others that removing a part of the legend is not implemented yet.
  I understand that adding another paramater to the mix implies the need of a legend for this parameter. Maybe I am using the wrong command to influence line size just for visual reasons. </p>
  
  <p>Thx for any suggestions!</p>