Rroot locus

apwidth=800;
  apheight=600;

npoles=2;
  poles[0]= [ 1,0];
  poles[1]= [ 1,0];
  nzeros=1;
  zeros[0]= [-1,0];

var dz=.04;
  var z= [ 0,0];
  var segmid=[0,0];
  var dgain=[0,0];
  var oldgain=[0,0];
  var segment=[0,0];
  Cream=[255,255,239];

function Blob(x,c){
    Colour(c);
    BoxFill(x[0]-dz/2,x[1]+dz/2,x[0]+dz/2,x[1]-dz/2);
  }

InitGraph();
  ScaleWindow(-5,-2.4,1.4,2.4);
  DrawAxes();
  Showroots();
  circle();
  Colour(Black);
  for(z[1]=-2.38;z[1]<2.4;z[1]+=dz){
    segmid[1]=z[1];
    oldgain[1]=0;
    for(z[0]=-4.98;z[0]<1.4;z[0]+=dz){
      getgain(z);
      if((gain[1]*oldgain[1]<0)&&(gain[0]<0)){
        csub(dgain,gain,oldgain);
        getsegment();
        segmid[0]=z[0]-dz*gain[1]/dgain[1];
        LineStart(segmid[0]-segment[0]/2,segmid[1]-segment[1]/2);
        LineTo(segmid[0]+segment[0]/2,segmid[1]+segment[1]/2);
      }
      copy(gain,oldgain);
    }
  }
  for(z[0]=-4.98;z[0]<1.4;z[0]+=dz){
    segmid[0]=z[0];
    oldgain[1]=0;
    for(z[1]=-2.38;z[1]<2.4;z[1]+=dz){
      getgain(z);
      if((gain[1]*oldgain[1]<0)&&(gain[0]<0)){
        csub(dgain,gain,oldgain);
        getsegment();
        segmid[1]=z[1]-dz*gain[1]/dgain[1];
        LineStart(segmid[0]+segment[1]/2,segmid[1]-segment[0]/2);
        LineTo(segmid[0]-segment[1]/2,segmid[1]+segment[0]/2);
      }
      copy(gain,oldgain);
    }
  }
  Label('Unit circle',.2,.5);