title: Cycling through point shapes when more than 6 factor levels
body: <p>When using the aesthetic mapping <code>shape</code> within <code>geom_point</code>, I get the following message when the number of factors present exceeds 6:</p>
  
  <p>"The shape palette can deal with a maximum of 6 discrete values because more than 6 becomes difficult to discriminate; you have 15. Consider specifying shapes manually. if you must have them."</p>
  
  <p>I tend to agree with the principle of limiting the number of distinct shapes, however when <em>using shape in combination with color</em> this should not be a problem.</p>
  
  <p>Is there an elegant way to get ggplot to cycle through shapes, i.e. setting symbol7 = symbol1 etc? Right now it simply omits the points with factor level > 6.</p>