import graph;
unitsize(1cm);

ylimits(min=-3,max=4);

yaxis("$y_1$");

yaxis(axis=XEquals(2.2),"$y_2$",Ticks());

yaxis(axis=XEquals(4.4),"$y_3$",Ticks(modify=NoZero));

yaxis(axis=XEquals(6.6),"$y_4$",RightTicks(Step=2));

shipout(bbox(2mm,Fill(white)));