When we draw an arrow, which is roughly a curved strip, there is a sensible way to choose an ellipse which just about covers it. When we use these ellipses as proxies for the arrows (so we place the ellipses, then draw the arrows that they contain) we would want to have some extra spacing around the arrows, so around their proximal ellipses.

At present, we have a simple mechanisms for controlling this spacing, the --margin option. The argument is linear one: one specifies the margin (in the major and minor directions) for a zero arrow, and the rate at which the margin grows with arrow size. These parameters affect the plot quite dramatically: when the zero-arrow margins are small, regions with small magnitude contain many small arrows; When the rate is positive, larger arrows are well-spaced from each other, when negative larger arrows are congested. There are no hard-and-fast rules here, you just need to experiment (possibly also adjusting the --scale too) to get something that looks reasonable.