title: error : ggplot2 doesn't know how to deal with data of class uneval
body: <p>I have this code</p>
  
  <pre><code>ggplot() + 
    stat_density(kernel = "biweight",aes(x=fd, colour=id), data=foo1,position="identity",geom="line")+
    coord_cartesian(xlim = c(0, 200))+
    xlab("Flood Duration")+
    ylab("Density")+
    ggtitle("PDFs of Flood Duration")+
    ggsave("pdf_fd_conus.png")
  </code></pre>
  
  <p>And I wrote this function</p>
  
  <pre><code>pdf.plot&lt;-function(data,x,xl,yl,title,save){
    ggplot() + 
      stat_density(data, kernel = "biweight",aes_string(x=x, colour='id'),
                   position="identity",geom="line")+
      coord_cartesian(xlim = c(0, 200))+
      xlab(xl)+
      ylab(yl)+
      ggtitle(title)+
      ggsave(save)
  }
  </code></pre>
  
  <p>Calling using this:</p>
  
  <pre><code>pdf.plot(data=foo1,x='fd',xl='b',
            yl='a',title='a',save='y.png')
  </code></pre>
  
  <p>But I am getting this error:</p>
  
  <pre><code>Error: ggplot2 doesn't know how to deal with data of class uneval
  Called from: eval(expr, envir, enclos)
  </code></pre>
  
  <p>This is dput(head(foo1,4))</p>
  
  <pre><code>structure(list(id = structure(c(1L, 1L, 1L, 1L), .Label = c("dfa", 
  "dfb", "cfa", "csb", "bsk"), class = "factor"), lon = c(-70.978611, 
  -70.978611, -70.945278, -70.945278), lat = c(42.220833, 42.220833, 
  42.190278, 42.190278), peakq = c(14.7531, 17.3865, 3.3414, 2.7751
  ), area = c(74.3327, 74.3327, 11.6549, 11.6549), fd = c(29, 54.75, 
  23, 1), tp = c(14.25, 19.75, 13.5, 0.5), rt = c(14.75, 35, 9.5, 
  0.5), bl = c(15485.3, 15485.3, 8242.64, 8242.64), el = c(0.643551, 
  0.643551, 0.474219, 0.474219), k = c(0.325279, 0.325279, 0.176624, 
  0.176624), r = c(81.947, 81.947, 38.7003, 38.7003), si = c(0.0037157, 
  0.0037157, -9999, -9999), rr = c(0.00529193, 0.00529193, 0.00469513, 
  0.00469513)), .Names = c("id", "lon", "lat", "peakq", "area", 
  "fd", "tp", "rt", "bl", "el", "k", "r", "si", "rr"), row.names = c(NA, 
  4L), class = "data.frame")
  </code></pre>
  
  <p>Could you please help?</p>