title: Dataframe issue with NetworkD3 in R
body: <p>I'm getting a</p>
  
  <pre><code>Error in `[.data.frame`(Links, , Source) : undefined columns selected
  </code></pre>
  
  <p>when I try to plot a network using the <code>forceNetwork</code> function in <code>NetworkD3</code>.</p>
  
  <p>My dataframes look like the following:</p>
  
  <pre><code>&gt; head(linkDf)
    Source Target
  1     25      1
  2     83      1
  3     83      2
  4     42      3
  5     26      4
  6     25      4
  </code></pre>
  
  <p>and</p>
  
  <pre><code> &gt; head(nodeDf2)     
           name group size
      1    A     1    2
      2    B     1    1
      3    C     2    1
      4    D     3    2
      5    E     2    2
      6    F     1    1
  </code></pre>
  
  <p>I'm trying to plot the network using the following function:</p>
  
  <pre><code>forceNetwork(Links = linkDf, Nodes = nodeDf2,
                Source = linkDf$Source, Target = linkDf$Target,
                NodeID = nodeDf2$name,
                Group = nodeDf2$group, opacity = 0.8)
  </code></pre>
  
  <p>All the columns have data in them, and I'm not sure why R tells me that I'm selecting undefined columns.</p>