I like this. I like that you looked at how everyone else did something (i.e. ordering by time) and asked "why?". Why not plot it in a more meaningful space? I have one idea -- what if node color was a function of time, and the x-axis was changed to effort? You start the node at one color, and progressively change it to another color using linear interpolation. When creating a TODO item:

  1. Specify title.
  2. Specify dueDate.
  3. Capture startDate = now().

Then

  currentColor = lerp(now(), startDate, dueDate, initColorVector, finalColorVector)

This would add a third dimension of information. I'd be able to see impact/effort as well as urgency visually. Another idea is to specify the shape (square, circle, triangle) to add some categorization capability. I'm not sure how much is too much. But the visual cortex is a beast.