layout scalebar
  code metapost
    # definition scalebar
    def s_scalebar (expr l, units, txt) =
      # scale for the scalebar
      begingroup
        interim warningcheck:=0;
        tmpl:=l / (Scale) * tmpscalebar * cm * units / 2;
        tmpx:=l / (Scale) * tmpscalebar * cm * units / 5;
        tmph:=5bp*tmpscalebar; % bar height
      endgroup;
      pickup PenC;
      thdraw (-tmpl,0)--(tmpl,0)--(tmpl,-tmph)--(-tmpl,-tmph)--cycle;
      p:=(0,0)--(tmpx,0)--(tmpx,-tmph)--(0,-tmph)--cycle;
      for i:=-2.5 step 2 until 2:
        fill p shifted (i * tmpx,0);
      endfor;
      begingroup
        interim labeloffset:=3.5bp;
        for i:=0 step (l/5) until (l-1):
          tmpx:=tmpl * (i * 2 / l - 1);
          label.bot(thTEX(decimal (i*tmpscalebar)) scaled tmpscalebar,(tmpx,-tmph-(1*tmpscalebar)));
        endfor;
        label.bot(thTEX(decimal (l*tmpscalebar) & "\thinspace" & txt) scaled tmpscalebar,(tmpl,-tmph-(1*tmpscalebar)));
        label.top(thTEX("1 : " & decimal round(Scale*tmpscalebar*100)) scaled tmpscalebar,(0,1*tmpscalebar));
        j:=((tmpscalebar-1) div 2)+1;
        %label.bot(thTEX("B.C.R.A. Grade 3B") scaled j,(0,-tmph*4));
        label.bot(thTEX("B.C.R.A. Grade 3B") scaled tmpscalebar,(0,-tmph*4));
      endgroup;
    enddef;
  endcode
endlayout