"crowding of offspring"... so don't think of it as a static graphic. Think of it as a miniature website of its own. It takes not too much effort to make a flat graphic into an image map. Put a set of destinations and 'tooltips' on each needed link. Viewer hovers over something, say a given union. All but the focal offspring of that union are hidden on the chart, and show up listed in the tooltip. Click there, and you get a new, derivative graphic, which *does* show that generation of that branch in full, relegating the others to single-line obscurity. <engineer waves hand airily> "that's the idea, carrying it out is mere turning the crank."

Slick map, btw - I like it.