The bubble color represents the country that detonated the nuclear explosion. The bubble size represents the yield of explosion, ranging from 0.0002 to 50,000 kiloton. The middle point is taken if the yield is estimated in a range. Explosions with unknown yield are represented by bubbles with the same size as those in the lowest group.
Source: Data until 1998 is from SIPRI. Data after 1998 is compiled from media reports.