Margin to leave around nodes when removing node overlap.
This guarantees a minimal non-zero distance between nodes.
If the attribute begins with a plus sign
'+', an additive margin is
specified. That is,
"+w,h" causes the node’s bounding box to be increased by
w points on the left and right sides, and by
h points on the top and bottom.
Without a plus sign, the node is scaled by
1 + w in the x coordinate and
1 + h in the y coordinate.
If only a single number is given, this is used for both dimensions.