title: graphing multiple data series in R ggplot
body: <p>I am trying to plot (on the same graph) two sets of data versus date from two different data frames. Both data frames have the same exact dates for each of the two measurements. I would like to plot these two sets of data on the same graph, with different colors. However, I can't get them on the same graph at all. R is already reading the date as date. I tried this:</p>
  
  <pre><code>qplot( date , NO3, data=qual.arn) 
  + qplot( qual.arn$date , qual.arn$DIS.O2, "O2(aq)" , add=T)
  </code></pre>
  
  <p>and received this error.</p>
  
  <pre><code>Error in add_ggplot(e1, e2, e2name) : 
    argument "e2" is missing, with no default
  </code></pre>
  
  <p>I tried using the ggplot function instead of qplot, but I couldn't even plot one graph this way.</p>
  
  <pre><code>ggplot(date=qual.no3.s, aes(date,NO3))
  
  Error: ggplot2 doesn't know how to deal with data of class uneval
  </code></pre>
  
  <p>PLEASE HELP. Thank you!</p>