Suppose your data for line 1 is (x1,y1), (x2,y2) and for line 2 is (x3,z1), (x4,z2) (with as many data points as needed).

Make your x data [x1,x2,x3,x4], y data [y1,y2,null,null] and z data [null,null,z1,z2].

Then under options put spanGaps: true.