Specifies how much, in inches, to extend the drawing area around the minimal area needed to draw the graph.
pad is a single double, both the x and y pad values are set
equal to the given value. This area is part of the
drawing and will be filled with the background color, if appropriate.
Normally, a small
pad is used for aesthetic reasons, especially when
a background color is used, to avoid having nodes and edges abutting
the boundary of the drawn region.