title: ggplot2 - geom_ribbon bug?
body: <p>This code throws an error and I can't figure out why...</p>
  
  <pre><code>library( plyr )
  library( ggplot2 )
  library( grid )
  library( proto )
  
  # the master dataframe
  myDF = structure(list(Agg52WkPrceRange = c(2L, 2L, 2L, 2L, 2L, 2L, 3L, 
  5L, 3L, 5L, 3L, 5L, 3L, 2L, 3L, 3L, 3L, 3L, 2L, 2L, 2L, 2L, 2L, 
  3L, 3L, 3L, 3L, 3L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
  3L, 3L, 3L, 4L, 3L, 4L, 3L, 4L, 4L, 4L, 4L), OfResidualPntReturn52CWk = c(0.201477324, 
  0.22350293, 0.248388728, 0.173871456, 0.201090654, 0.170666183, 
  0.18681883, 0.178840521, 0.159744891, 0.129811042, 0.13209741, 
  0.114989407, 0.128347625, 0.100945992, 0.057017002, 0.081123718, 
  0.018900252, 0.021784814, 0.081931816, 0.059067844, 0.095879746, 
  0.038977508, 0.078895248, 0.051344317, 0.077515295, 0.011776214, 
  0.099216033, 0.054714439, 0.022879951, -0.079558277, -0.050889584, 
  -0.006934821, -0.003407085, 0.032545474, -0.003387139, 0.030418511, 
  0.053942523, 0.051398537, 0.073482355, 0.087963039, 0.079555591, 
  -0.040490418, -0.130754663, -0.125826649, -0.141766316, -0.150708718, 
  -0.171906882, -0.174623614, -0.212945405, -0.174480554), IndependentVariableBinned = structure(c(1L, 
  1L, 1L, 1L, 1L, 2L, 3L, 10L, 3L, 10L, 4L, 10L, 4L, 2L, 4L, 4L, 
  4L, 5L, 2L, 2L, 2L, 3L, 3L, 5L, 5L, 5L, 5L, 6L, 3L, 6L, 6L, 6L, 
  6L, 7L, 7L, 7L, 7L, 7L, 8L, 8L, 8L, 8L, 9L, 8L, 9L, 9L, 9L, 9L, 
  10L, 10L), .Label = c("1", "2", "3", "4", "5", "6", "7", "8", 
  "9", "10"), class = "factor")), .Names = c("Agg52WkPrceRange", 
  "OfResidualPntReturn52CWk", "IndependentVariableBinned"), row.names = 28653:28702, class = "data.frame")
  
  # secondary data frame
  meansByIndependentVariableBin = ddply( myDF , .( IndependentVariableBinned ) , function( df ) mean( df[[ "OfResidualPntReturn52CWk" ]] ) )
  
  # construct the plot
  thePlot = ggplot( myDF , aes_string( x = "IndependentVariableBinned" , y = "OfResidualPntReturn52CWk" ) )
  thePlot = thePlot + geom_point( data = meansByIndependentVariableBin , aes( x = IndependentVariableBinned , y = V1 ) )
  thePlot = thePlot + geom_line( data = meansByIndependentVariableBin , aes( x = IndependentVariableBinned , y = V1 , group = 1 )  )
  thePlot = thePlot + geom_ribbon( data = meansByIndependentVariableBin , aes( group = 1 ,  x = IndependentVariableBinned , ymin = V1 - 1 , ymax = V1 + 1 ) )
  
  # print - error!
  print( thePlot )
  </code></pre>
  
  <p>I've tried with/without group=1.  The error is:</p>
  
  <pre><code>Error in eval(expr, envir, enclos) : 
    object 'OfRelStrength52CWk' not found
  </code></pre>
  
  <p>but not sure how that is relevant??  I must be missing something obvious. Take away the last geom (ribbon) and it plots just fine!</p>