title: How do you build a graph from a data frame using the igraph package?
body: <p>Any way to build a graph ( igraph package ) from a Data Frame in R ?</p>
  
  <p>The data frame contains node relationships.</p>
  
  <pre><code>    df = data.frame ( 
  A = c("Berlin", "Amsterdam", "New York") , 
  B = c("Munich", "Utrecht", "Chicago") 
  ) 
  </code></pre>
  
  <p>Edges are : </p>
  
  <pre><code>Berlin -&gt; Munich
  Amsteram -&gt; Utrecht
  New York -&gt; Chicago
  </code></pre>