Rroot locus

npoles=3;
  poles[0]= [ 0,0];
  poles[1]= [ 0,0];
  poles[2]= [-3,0];
  nzeros=1;
  zeros[0]= [-1/3,0];

var zoom=2;
  var ds=.02*zoom;

var segmid=[0,0];
  var dgain=[0,0];
  var oldgain=[0,0];
  var segment=[0,0];

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

function Blip(x,c){
    Colour(c);
    BoxFill(x[0]-ds/4,x[1]+ds/4,x[0]+ds/4,x[1]-ds/4);
  }
  function Showroots(){
    for(i=0;i<npoles;i++){
      Blob(poles[i],Black);
    }
    for(i=0;i<nzeros;i++){
      Blob(zeros[i],Black);
      Blip(zeros[i],White);
    }
  }
  function getsegment(){
    mod=Math.sqrt(dgain[0]*dgain[0]+dgain[1]*dgain[1]);
    if(mod>0){
      segment[0]=ds*dgain[0]/mod;
      segment[1]=-ds*dgain[1]/mod;
    }else{
      segment=[0,0];
    }
  }
  InitGraph();
  ScaleWindow(-1.6*zoom,-zoom,1.6*zoom,zoom);
  DrawAxes();
  Showroots();
  Colour(Black);
  for(s[1]=-zoom+ds/2;s[1]<zoom;s[1]+=ds){
    segmid[1]=s[1];
    oldgain[1]=0;
    for(s[0]=-1.6*zoom;s[0]<1.6*zoom;s[0]+=ds){
      getgain(s);
      if((gain[1]*oldgain[1]<0)&&(gain[0]<0)){
        csub(dgain,gain,oldgain);
        getsegment();
        segmid[0]=s[0]-ds*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(s[0]=-1.6*zoom;s[0]<1.6*zoom;s[0]+=ds){
    segmid[0]=s[0];
    oldgain[1]=0;
    for(s[1]=-zoom+ds/2;s[1]<zoom;s[1]+=ds){
      getgain(s);
      if((gain[1]*oldgain[1]<0)&&(gain[0]<0)){
        csub(dgain,gain,oldgain);
        getsegment();
        segmid[1]=s[1]-ds*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('Root locus',.5*zoom,.5*zoom);