Also, this is a great problem, and we tried to solve it in a more general context. http://dpd.cs.princeton.edu/Papers/DGKN97.pdf

I believe it's an open problem what makes a curve look "natural" or how to do a good job of routing multiple splines around obstacles so they do not intersect each other, though the work of Keean Crane at CMU seems promising: https://www.cs.cmu.edu/~kmcrane/Projects/RepulsiveCurves/ind...