The people who set it and the zero baseline is set according to this:
"Instrumental global surface temperature record since widespread reliable measurements began in the late 19th century; see also [1] The X axis represents time, and the Y axis represents the temperature anomaly (departure from 1961-1990 baseline of 14.0°C, Jones et al. 1999)."
I gave the source as Wikipedia but not the link. Here is the actual Wikipedia link.
