graph vertex labelling

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I have a graph (poset actually) that has a rational function labelling each vertex. I would like to show just the edges and the vertex labels. If I try to set vertex_shape='None' I get a crash deep in matplotlib. If I try anything else I get the marker symbol printed over the label. For example

  P = RootSystem(['A',3]).root_poset()
  P.show(vertex_color='white')

Will show the graph with circles over the labels. If I shrink the labels, then the edges cover the labels. If I do

  P = RootSystem(['A',3]).root_poset()
  P.show(vertex_color='white', vertex_shape='None')

there is a strange crash.