So weird "off by 1" in the display code ?

(e.g.: trying to link one more point on the spline than there are data points, and thus having something with weird coordinates)

(that could typically happen in bicubic smoothing routine : to draw a spline between "point(0)" and "point(1)", you also need "point(-1)" and "point(2)" for the bicubic computations. Those might not even exist in the data array).