A single year-on-year comparison, this year versus last, is one of the most common ways a figure gets reported, and also one of the most easily distorted, since the comparison depends heavily on what happened to be true in that one particular prior year chosen as the baseline.
If last year happened to be unusually good, this year will look disappointing by comparison even if it is still perfectly healthy by any longer historical standard, and the reverse is equally true if last year happened to be an unusually poor one used as the point of reference.
A longer chart, spanning five or ten years rather than just two, tends to smooth out this kind of distortion, revealing whether a single year's decline is part of a genuine trend or simply a return to normal after an unusually strong prior comparison point. The value tends to show up later, often well after the initial effort has been made.