title: Python: igraph - set vertexseq none value to a specific one
body: <p>I want to learn more about igraph libary. I want to make a simple graph with two nodes labeled at different time. In igraph <a href="https://igraph.org/python/tutorial/latest/tutorial.html" rel="nofollow noreferrer">tutorial</a> I have found its possible to label the list at all after I generated the graph. It works with code:</p>
  <pre><code>from igraph import *
  
  p = Graph()
  p.add_vertices(1)
  p.vs[&quot;label&quot;] = [&quot;test&quot;, &quot;test1&quot;]
  layout = p.layout(&quot;kk&quot;)
  plot(p, layout=layout)
  </code></pre>
  <p>but what, if I want to label the graph step by step. That means I first want add some vertices then label it. Then perform some calculation add some other vertices and label that other on after I added it without loosing the first ones. I just tried it with following code, but for some reason it does not work as I want. How can I solve it?</p>
  <pre><code>from igraph import *
  
  p = Graph()
  p.add_vertices(1)
  p.vs[&quot;label&quot;] = [&quot;test&quot;]
  
  p.add_vertices(1)
  p.vs[&quot;label&quot;][1] = &quot;test1&quot;
  layout = p.layout(&quot;kk&quot;)
  plot(p, layout=layout)
  </code></pre>