Are you sure? The homepage says "Scroll, zoom and click on individual stations to see how passenger numbers have changed since 1997". I think it's for the whole period.

Many stations are white at the end, but have no reason for the increase to have been in the final year. Picking one I used to travel through [1], it's white on the map, last year's increase was <10%, but the increase since 1997 is huge.

(Also, this is Great Britain, since Northern Ireland isn't included.)

[1] https://en.wikipedia.org/wiki/Barnes_Bridge_railway_station