title: Error in eval(expr, envir, enclos) : object 'Tribe' not found
body: <p>I am following some online examples for getting to grips with ggplot2 in R. As part of this I am trying to add in the automatic coloring of series using pre-defined color schemes.</p>
  
  <p>The following code will work:</p>
  
  <pre><code>ggplot(subset(homes, STATE %in% c("MA","TX"))) + geom_point(aes_string(x="Date", y="Home.Value"))
  </code></pre>
  
  <p>However, when I add the color option:</p>
  
  <pre><code>ggplot(subset(homes, STATE %in% c("MA","TX"))) + geom_point(aes_string(x="Date", y="Home.Value"), color=State)
  </code></pre>
  
  <p>I get the error: <strong>"Error in layer(data = data, mapping = mapping, stat = stat, geom = GeomPoint,  : 
    object 'State' not found"</strong></p>
  
  <p>I have also tried 'Tribe' and 'cp1' with no luck and the same error message. Is there anything I need to do to bring the auto color options into the software so the objects are recognised?</p>
  
  <p>Thanks!</p>