title: How to melt R data.frame and plot group by bar plot
body: <p>I have following R data.frame: </p>
  
  <pre><code>  group match unmatch unmatch_active match_active
  1   A    10       4              0            0
  2   B   116      20              0            3
  3   c   160      27              1            4
  4   D    79      17              0            3
  5   E   309      84              4           14
  6   F   643     244             10           23
  ...
  </code></pre>
  
  <p>My goal is to plot a group by bar plot (<a href="http://www.cookbook-r.com/Graphs/Bar_and_line_graphs_(ggplot2)/" rel="nofollow noreferrer">http://www.cookbook-r.com/Graphs/Bar_and_line_graphs_(ggplot2)</a>/ section-Graphs with more variables) as shown in the link. </p>
  
  <p>I realize that before getting to that I need to get the data in to following format</p>
  
  <pre><code>  group variable value
  1   A    match    10
  2   B    match   116
  3   C    match   160
  4   D    match    79
  5   E    match   309
  6   F    match   643
  7   A    unmatch   4
  8   B    unmatch  20
  ...
  </code></pre>
  
  <p>I used the melt function: </p>
  
  <pre><code>groups.df.melt &lt;- melt(groups.df[,c('group','match','unmatch', 'unmatch_active', 'match_active')],id.vars = 1)
  </code></pre>
  
  <p>I don't think I am doing the melt correctly because after I execute above groups.df.melt has 1000+ lines which doesn't make sense to me. </p>
  
  <p>I looked at how <a href="https://stackoverflow.com/questions/22264744/draw-histograms-per-row-over-multiple-columns-in-r/22264947#22264947">Draw histograms per row over multiple columns in R</a> and tried to follow the same yet I don't get the graph I want. </p>
  
  <p>In addition I get following error: When I try to do the plotting: </p>
  
  <pre><code>ggplot(groups.df.melt, aes(x='group', y=value)) + geom_bar(aes(fill = variable), position="dodge") + scale_y_log10()
  
  Mapping a variable to y and also using stat="bin".
    With stat="bin", it will attempt to set the y value to the count of cases in each group.
    This can result in unexpected behavior and will not be allowed in a future version of ggplot2.
    If you want y to represent counts of cases, use stat="bin" and don't map a variable to y.
    If you want y to represent values in the data, use stat="identity".
    See ?geom_bar for examples. (Deprecated; last used in version 0.9.2)
  Error in pmin(y, 0) : object 'y' not found
  </code></pre>