import graph;
unitsize(1cm);

xlimits(min=-3,max=3);

xaxis("$x_1$");

xaxis(axis=YEquals(-2),
      Label("$x_2$",align=N),
      Ticks());

xaxis(axis=YEquals(-4),
      Label("$x_3$",position=EndPoint, align=N),
      Ticks(modify=NoZero));

xaxis(axis=YEquals(-6),
      Label("$x_4$",position=EndPoint, align=NE),
      RightTicks());

xaxis(axis=YEquals(-8),
      Label("$x_5$",position=BeginPoint),
      LeftTicks(Step=2));

xaxis(axis=YEquals(-10),
      Label("$x_6$",position=BeginPoint, align=NW),
      LeftTicks(Step=1.5,step=.5,endlabel=false));

shipout(bbox(3mm,Fill(white)));