Computes a control point perpendicular to the line between two nodes, used for xspline() curve generation.
perp_mid(x0, y0, x1, y1, cex, q = 0.5)List with x, y coordinates of control point.
Start x coordinate.
Start y coordinate.
End x coordinate.
End y coordinate.
Curvature amount (positive = left, negative = right).
Position along edge (0 = start, 0.5 = middle, 1 = end).