In each graph each pixel represents the average of five elections of 10000 voters electing two seats from four choices. The full color graph is the result of blending the primary colors representing the two winners.
"Combinatoric" variants of single winner election methods adapt to multiwinner situations by enumerating all possible winning sets of the available choices and using a simulated voter's preferences on the choices in each set to determine a preference for each winner-set. Voting on the n-choose-k preferences for winner-sets then procedes as for a single-winner election.
STV used here gives every voter 1.0 vote which is fractionated when used to elect a choice that is over quota. A fraction of every vote for a winner is transferred to lower ranked choices of those voters.