title: networkx add_node with specific position
body: <p>I am still a beginner with networkx
  I want to add multiple types of nodes in different position, I used the following code </p>
  
  <pre><code>pos = {0: (40, 20), 1: (20, 30), 2: (40, 30), 3: (30, 10)} 
  X=nx.Graph()
  nx.draw_networkx_nodes(X,pos,node_size=3000,nodelist=[0,1,2,3],node_color='r')
  </code></pre>
  
  <p>but when I want to access the Graph X , if I type X.node it returns an empty list
  and if I want to add more nodes I have to set their positions in the beginning using pos dictionary. </p>
  
  <p>How can I  add  nodes to a graph in a specific location x and y using add_node()</p>