title: Combine (rbind) data frames and create column with name of original data frames
body: <p>I have several data frames that I want to combine by row. In the resulting single data frame, I want to create a new variable identifying which data set the observation came from.</p>
  
  <pre><code># original data frames
  df1 &lt;- data.frame(x = c(1, 3), y = c(2, 4))
  df2 &lt;- data.frame(x = c(5, 7), y = c(6, 8))
  
  # desired, combined data frame
  df3  &lt;- data.frame(x = c(1, 3, 5, 7), y = c(2, 4, 6, 8),
                     source = c("df1", "df1", "df2", "df2")
  # x y source
  # 1 2    df1
  # 3 4    df1
  # 5 6    df2
  # 7 8    df2
  </code></pre>
  
  <p>How can I achieve this?
  Thanks in advance!</p>