This is the actual graph of 'Wu Ji', a Chinese concept for Primordial Universe: the state before everything was shared. The circle symbolise the infinity, integrity, indivisibility of the universe. 
Wuji - Taiji - Yin/yang - The four phenomena - Bagua - Trigrams

This is one more of hundreds of circles in the Wikipedia; another SVG drawing, consisting of a black stroke with white filling
Own work
水水