Motion in one dimension

As we know v = change in position / change in time = y2  y1 / t2  t1
But In the diagram, I am confused about the position of the 'y2' and 'y1'. It should be y1 in the place of y2 and y2 in the place of y1. Please help.

@indranil
Yes... This graph is mathematically not correct. At point A corresponding time is t1 and corresponding position must be y1. While at point B corresponding time is t2 and corresponding position will be y2