How do I union graph properties?

Question

I have two graphs

g1 = Graph[{"1", "2", "3"}, {{2, 1}, {3, 1}},VertexLabels -> {"1" -> Placed["One", Tooltip], "2" -> Placed["Two", Tooltip], "3" -> Placed["Three", Tooltip]}]

and

g2 = Graph[{"5", "2", "3"}, {{2, 1}, {3, 1}}, VertexLabels -> {"5" -> Placed["Five", Tooltip], "2" -> Placed["Two", Tooltip], "3" -> Placed["Three", Tooltip]}]

Each of which displays the appropriate label when doing a mouseover.

Then I do

GraphUnion[g1,g2]

and get the desired graph, but the tooltips for the vertex names are gone. If I do

GraphUnion[g1,g2,Options[g1]] 

then as expected I get the tooltips for the vertices of g1. But how can I get the tooltips for ALL the vertices (i.e. vertices for both graphs)?

Thanks much!

Answer 1

You can extract the VertexLabels from both graphs and join them together, there might be a better way but this works

g1 = Graph[{"1", "2", "3"}, {{2, 1}, {3, 1}},
  VertexLabels -> {"1" -> Placed["One", Tooltip],
                   "2" -> Placed["Two", Tooltip],
                   "3" -> Placed["Three", Tooltip]}];
g2 = Graph[{"5", "2", "3"}, {{2, 1}, {3, 1}},
  VertexLabels -> {"5" -> Placed["Five", Tooltip],
                   "2" -> Placed["Two", Tooltip],
                   "3" -> Placed["Three", Tooltip]}];
GraphUnion[g1, g2, 
 VertexLabels -> Flatten[VertexLabels /. {Options[g1], Options[g2]}]]

Look at:

Flatten[VertexLabels/.{Options[g1],Options[g2]}]
(* {2->Placed[Two,Tooltip],1->Placed[One,Tooltip],
   3->Placed[Three,Tooltip],2->Placed[Two,Tooltip],
   5->Placed[Five,Tooltip],3->Placed[Three,Tooltip]} *)

And you see it has all the options, the repeated elements could be removed by using Union