'I created an ordering based on the length of the lines, so that longer lines were drawn “behind” the shorter, more local lines. Then I used colorRampPalette() to generate a color palette from black to blue to white, and colored the lines according to order they were drawn.'

If I am understanding this correctly (I'm probably not), longer lines would never be as bright as short lines. Why? Let's say hypothetically, that all Bostonians had at least 10 friends in Madrid, then wouldn't we want to see that as a white line on the chart? Please correct me if I am wrong.