A few line designs. First up, a layout in Scapple:
And full black lines with all lines showing, but the lines are aligned to horizontal edges:
And there is a mouse over design for mouse over the top right node showing ONLY the actively linked lines:
And finally, showing the other lines lightly in the background, as an alternative (I prefer the previous though):
Examples of ctrl-clicking to control the headings:
This screenshot of a Liquid View style interaction from Scapple highlights how there can be too many lines and how the lines need to be able to snap to a side, in this case here it would be useful if the lines could snap to the horizontal sides of the headings so that ‘My Work’ and ‘Knowledge Work’ could be more clearly linked to the Process to Augment.
As for the issue with too many lines, a method of allowing the user to click on headings and have those lines more strongly visible is an option (cmd-click or shift-click to select more than one, as per macOS convention). Notice how a whole column is selected, not just specific sub-heads: