Displaying Chessboard Graphs

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I'm looking at various graphs and their plotting methods. For the code

G = graphs.KnightGraph([3,3])
G.show()

Sage displays:

I'd like the vertices to be arranged in a 3 by 3 grid so that the chessboard nature of the graph is more apparent. From the documentation here I tried specifying the position by using a dictionary:

G = graphs.KnightGraph([3,3])
pos_dict = {}
for i in range(0,2):
    for j in range(0,2):
        pos_dict[G.vertices()[3*i+j]] = [i*.5,j*.5]
pl = G.graphplot(pos=pos_dict)
pl.show()

This doesn't give the intended result. Even stranger, running the code multiple times shows the position of only some of the vertices is fixed while others change position.

How can I display an n by n chessboard graph so that it's vertices form an n by n square?