[coda_]

That's what I thought as well... But I think what the diagram isn't showing is that the drops are actually moving horizontally to the left at the speed that the jug was moving at the time the drop left the jug. This means each one is moving to the left slightly faster than the one before it... And as they fall this creates an increasing horizontal distance between them... The line is kept at a constant angle due to the fact that the earlier drops are falling downward slightly faster than the later drops. The horizontal and vertical difference between the adjacent drops maintains a 1 to 1 ratio.