add a grid to any Graphviz graph

addGrid.gvpr

/**************************************************************************

add a grid to a graph in .dot format

**************************************************************************/
BEGIN{
  int     i, Indx, bcnt, LR;
  string  Type, Val, Gcolor[], Gstyle[], Gsize[], tmpstr;
  string  Hcolor, Vcolor, Hstyle, Vstyle, gridType, gridAlign;
  float   Hsize, Vsize, deltaX, deltaY, minX, minY, maxX, maxY;
  string  help="TBD";
  float   rankPos[];
  node_t  LL, UR, aNode;
  edge_t  E;
  graph_t Root;

help="
  adddGrid.gvpr - add a grid to any Graphviz graph

  options:
    grid=horizontal|vertical|both  - set direction(s) of grid lines
    size=rank     - grid lines run through center of ranks
    [GHV]size=    - set gridlines to numeric value (123in for inches or 123pt for points),
                    fraction (4 for 1/4th of graph size) or % (10% for 10% of graph size)
    [GHV]style=   - set grid style to valid Graphviz style string
    [GHV]color=   - set grid color to rotating value from provided list
    note:   G|H|V prefix indicates set attribute for horizontal, vertical or both(G)

  usage examples:
    dot myFile.gv | gvpr -cf addGrid.gvpr |neato -n2 ...
    dot myFile.gv | gvpr -a 'size=rank Gcolor=green,purple,red grid=horizontal' -cf addGrid.gvpr |neato -n2 ...
";
////////////////////////////////////////////////////////////////////////////
  // do NOT call with direct output from sprintf - there is a bug - string will be empty
  void doErrs(string eString) 
{
    printf(2, "Error:: %s\n", eString);
}
////////////////////////////////////////////////////////////////////////////
//  float inch2pt(float i){
//    return 72.*i;
//  }
////////////////////////////////////////////////////////////////////////////
  float strInch2pt(string inches) {
    float f;
    sscanf(inches, "%lf", &f);
    //print("// strInch2pt: >", inches, "<  ", f, " ", f*72.);
    return f*72.;
  }
////////////////////////////////////////////////////////////////////////////
  string next(string list) {
    int    Cnt, nxt, save[];
    string retS, nxtTok[int];

    print("//  NEXT: ", save[list]);
    Cnt=tokens(list,nxtTok,",;");
    nxt=save[list];
    if(nxt>=(Cnt)) {
      save[list]=0;
      nxt=0;
    }
    retS=nxtTok[nxt];   //save[list]];
    save[list]++;
    return(retS);
  }
////////////////////////////////////////////////////////////////////////////
  float gridSizeChk(string sizeStr, float delta) {
    float size;

    size=72.;
    print("//  SIZE start: ", sizeStr, " delta: ", delta);
    if (sizeStr=="(+([0-9])pt)") {         // points
      size=(float)sub(sizeStr,"pt");
    } else if (sizeStr=="(+([0-9])in)") {  // inches
      size=72.*(float)sub(sizeStr,"in");
    } else if (sizeStr=="(+([0-9])%)") {   // percent 20% means 5 intervals
      size=delta * .01 * (float)sub(sizeStr,"%");
    } else if (sizeStr=="(+([0-9]))") {    // if bare number, divide by that number
      size=delta / (float)sizeStr;
    } else if (sizeStr=="rank") {          // align on rank
      size=-1;  // flag?
    } else {
      doErrs("bad gridsize attribute value \"" + sizeStr + "\"");
    }
    print("//  SIZE end: ", sizeStr, " size: ", size, " delta: ", delta);
    return size;
  }
////////////////////////////////////////////////////////////////////////////
  void straightLine(float x1, float y1, float x2, float y2, string C, string S, string lbl) {
    node_t n1, n2;
    string tmpStr;
    tmpstr="__grdNodeA__" + (string) x1 + "_" + (string) y1 +"_" + (string) x2 +"_" + (string) y2;
    n1=node(Root,tmpstr);
    if (lbl!="") {
      n1.label=lbl;
      n1.shape="plaintext";
    } else {
      n1.label="";
      n1.shape="point";
      n1.style="invis";
    }
    n1.pos=(string)x1 + "," + (string)y1;

    tmpstr="__grdNodeB__" + (string) x1 + "_" + (string) y1 +"_" + (string) x2 +"_" + (string) y2;
    n2=node(Root,tmpstr);
    n2.label="";
    n2.shape="point";
    n2.style="invis";
    n2.pos=(string)x2 + "," + (string)y2;
    E=edge(n1,n2,"");  // new edge,
    E.dir="none";
    E.color=C;
    E.style=S;
    E.penwidth=1.;
    tmpstr=sprintf("%.2f,%.2f %.2f,%.2f %.2f,%.2f %.2f,%.2f",
                   x1, y1, x1, y1, x2, y2, x2, y2);
    E.pos=tmpstr;
    //print("// straight: " + tmpstr);
  }
////////////////////////////////////////////////////////////////////////////
  void doGrid() {
    float n;
    int II;
    print("// check grid1 - ", gridType,"  LR: ",LR," gridAlign: ", gridAlign);
    II=0;
    if (gridType=="both" || gridType=="vertical" || (LR==1 && gridAlign=="rank")) {
      if (LR==1 && gridAlign=="rank") {
        for (rankPos[n]) {
          print("//  check g1a   - ", n);
          straightLine(n,minY,n,maxY, next(Vcolor), Vstyle, (string)++II);
        }
      } else {
        for (n=minX; n<=maxX; n+=Vsize) {
          print("//  check g1b   - ", n," - ",Vsize);
          straightLine(n,minY,n,maxY, next(Vcolor), Vstyle, (string)++II);
        }
      }
    }
    print("// check grid2");
    II=0;
    if (gridType=="both" || gridType=="horizontal" || (LR==0 && gridAlign=="rank")) {
      if (LR==0 && gridAlign=="rank") {
        forr (rankPos[n]) {
          straightLine(minX,n,maxX,n, next(Hcolor), Hstyle, (string)++II);
          print("//  n: ",n,"  ",II);
        }
      } else {
        for (n=maxY; n>=minY; n-=Hsize) {
          straightLine(minX,n,maxX,n, next(Hcolor), Hstyle, (string)++II);
          print("//  n: ",n,"  ",II);
        }
      }
    }
    print("// check g last");
  }
}
/////////////////////////////////////////////////////////////////////////////////
BEG_G{
  Root=$G;
  if (hasAttr(Root, "layout"))
    Root.layout="neato";
  Root.phase="";
  maxX=(float)xOf(urOf(Root.bb));
  minX=(float)xOf(llOf(Root.bb));
  deltaX=maxX-minX;
  maxY=(float)yOf(urOf(Root.bb));
  minY=(float)yOf(llOf(Root.bb));
  deltaY=maxY-minY;
  Hcolor="red";
  Vcolor="red";
  Hstyle="dashed";
  Vstyle="dashed";
  Hsize=gridSizeChk("72pt", 0.);
  Vsize=Hsize;

  gridType="both";
  Gcolor["both"]=Val;
  Gstyle["both"]=Val;
  Gsize["both"] =Val;
  i=0;
  while (i<ARGC) {
    print("// ARG >", ARGV[i],"<");
    Indx=1+index(ARGV[i],"=");
    Type=substr(ARGV[i],0,1);

    print("//  type: ", Type);
    Val=substr(ARGV[i],Indx);
    if (ARGV[i]=="[Gg]rid=*") {
      gridType=Val;
      print("// ",ARGV[i], "  ",Val,"  << gridType");
    } else  if (ARGV[i]=="[GHV]color=*") {
      if (Type=="H") {
        Hcolor=Val;
      } else if (Type=="V") {
        Vcolor=Val;
      } else {
        Hcolor=Val;
        Vcolor=Val;
      }
      print("// ",ARGV[i], "  << color");
    } else  if (ARGV[i]=="[GHV]style=*") {
      if (Type=="H") {
        Hstyle=Val;
      } else if (Type=="V") {
        Vstyle=Val;
      } else {
        Hstyle=Val;
        Vstyle=Val;
      }
      print("// ",ARGV[i], "  << style");
    } else  if (ARGV[i]=="size=rank|Gsize=rank") {
      gridAlign="rank";
    } else  if (ARGV[i]=="[GHV]size=*") {
      if (Type=="H") {
        Hsize=gridSizeChk(Val, deltaY);
      } else if (Type=="V") {
        Vsize=gridSizeChk(Val, deltaX);
      } else {
        Hsize=gridSizeChk(Val, deltaY);
        Vsize=gridSizeChk(Val, deltaX);
      }
      print("// ",ARGV[i], "  << size");
    } else {
      printf(2, "Bad argument: \"%s\"\n", ARGV[i]);
      printf(2, "%s\n", help);      
      exit (1);
    }
    i++;
  }
  //print("//   Vsize: ",Vsize);
  //print("//  bb: ",$G.bb, "  ", deltaX, "  ",deltaY);
  if (hasAttr(Root, "grid") && Root.grid!="") {
    tmpstr=tolower(Root.grid);
    if (tmpstr!="both" && tmpstr!="horizontal" && tmpstr!="vertical") {
      doErrs("grid attribute value \"" + Root.grid + "\" is illegal (horizontal, vertical, or both)");
      exit(8);
    }
    gridType=tmpstr;
  }
  //  set other grid values
  //  - interval (pts or inches) or count
  //  - horiz/vert color
  //  - horiz/vert line style

  print("// check 1");
  if (hasAttr(Root, "gridHcolor") && Root.gridHcolor!="") {
    Hcolor=Root.gridHcolor;
  }
  if (hasAttr(Root, "gridVcolor") && Root.gridVcolor!="") {
    Vcolor=Root.gridVcolor;
  }
  if (hasAttr(Root, "gridcolor") && Root.gridcolor!="") {
    Hcolor=Root.gridcolor;
    Vcolor=Root.gridcolor;
  }
  if (hasAttr(Root, "gridHstyle") && Root.gridHstyle!="") {
    Hstyle=Root.gridHstyle;
  }
  if (hasAttr(Root, "gridVstyle") && Root.gridVstyle!="") {
    Vstyle=Root.gridVstyle;
  }
  if (hasAttr(Root, "gridstyle") && Root.gridstyle!="") {
    Hstyle=Root.gridstyle;
    Vstyle=Root.gridstyle;
  }
  if (hasAttr(Root, "gridHsize") && Root.gridHsize!="") {
    Hsize=gridSizeChk(Root.gridHsize, deltaY);
  }
  if (hasAttr(Root, "gridVsize") && Root.gridVsize!="") {
    Vsize=gridSizeChk(Root.gridVsize, deltaX);
  }
  if (hasAttr(Root, "gridsize") && Root.gridsize!="") {
    Hsize=gridSizeChk(Root.gridsize, deltaY);
    Vsize=gridSizeChk(Root.gridsize, deltaX);
  }
  //print("// check 2 -- Vsize: ",Vsize);

  LR=0;
  if (hasAttr(Root,"rankdir")) {
    if (toupper(Root.rankdir)=="@(LR|RL)") {
      LR = 1;
    } else if (toupper(Root.rankdir)=="@(TB|BT)") {
      LR=0;
    }
  }
  //print("// check 3");
}
N{
  if (hasAttr($, "pos") && $.pos!="") {
    if (LR==1) {
      rankPos[$.X]=1;
    } else {
      rankPos[$.Y]=1;
    }
  }
}
END_G{
  //print("// check 4");
  doGrid();
}

unix.Ranks.png

Can we add the grid behind the actual nodes?

Try adding outputorder=edgesfirst to your graph (see https://www.graphviz.org/docs/attrs/outputorder/ & https://www.graphviz.org/docs/attr-types/outputMode/).
No guarantees, let me know if it works for you.

It works, thanks a lot!