title: igraph how to vary node size and border colour/thickness on condition
body: <p>for a dataframe </p>
  
  <pre><code>df &lt;- data.frame(g1=c("x1","x2","x2","x3","x4","x5","x5","x3"),
                                 g2=c("y1","y4","y2","y4","y3","y4","y5","y4"), 
                                 g1value=c(1,2,2,3,4,5,5,3),
                                 g2value=c(2,4,2,4,5,4,NA, 4),
                                 stringsAsFactors = FALSE)
  
  library(igraph)
  g=graph.data.frame(df, directed=F)
  plot(g)
  </code></pre>
  
  <p>how shall i specify the size of nodes such that each size of x-nodes correspond to g1value,and y nodes to g2value (ignore the node if its size is na)?</p>
  
  <p>and how to vary either node border colour or thickness on conditions, such as when the node is x2?</p>
  
  <p>If there're many nodes, and the plotting result is crowded, anyway to space it out?</p>
  
  <p>Thanks.</p>