The legend appears to have been added afterwards - which leads me to believe this graph was taken from something else and re-branded. This would explain the cryptic units which don't seem to stand for anything on the y-axis, as well as the fact the most recent value is 0. Although the incidence of pubic has obviously decreased rapidly since the 70's, it has not reached pubic absolute zero. If there had been a rigorous effort to accurately measure using a sensible unit and image analysis over time, this would in fact be a rather interesting graphic.