A cubic spline modification.
par1 = start x-value par2 = start y-value par3 = end y-value par4 = start dy/dx par5 = end dy/dx par6 = end x-value
par2 = start y-value
par3 = end y-value
par4 = start dy/dx
par5 = end dy/dx
par6 = end x-value
