Google map with route-trace loading data instead of adding it on click

In this map

http://econym.org.uk/gmap/example_snappath.htm

source:

<script type="text/javascript"> 
//<![CDATA[
if (GBrowserIsCompatible()) {

  var map = new GMap2(document.getElementById("map"));
  map.setCenter(new GLatLng(53.7877, -2.9832),13)
  map.addControl(new GLargeMapControl());
  map.addControl(new GMapTypeControl());
  var dirn = new GDirections();

  var firstpoint = true;
  var gmarkers = [];
  var gpolys = [];
  var dist = 0;


  GEvent.addListener(map, "click", function(overlay,point) {
    // == When the user clicks on a the map, get directiobns from that point to itself ==
    if (!overlay) {
      if (firstpoint) {
        dirn.loadFromWaypoints([point.toUrlValue(6),point.toUrlValue(6)],{getPolyline:true});
      } else {
        dirn.loadFromWaypoints([gmarkers[gmarkers.length-1].getPoint(),point.toU开发者_运维技巧rlValue(6)],{getPolyline:true});
      }
    }
  });

  // == when the load event completes, plot the point on the street ==
  GEvent.addListener(dirn,"load", function() {
    // snap to last vertex in the polyline
    var n = dirn.getPolyline().getVertexCount();
    var p=dirn.getPolyline().getVertex(n-1);
    var marker=new GMarker(p);
    map.addOverlay(marker);
    // store the details
    gmarkers.push(marker);
    if (!firstpoint) {
      map.addOverlay(dirn.getPolyline());
      gpolys.push(dirn.getPolyline());
      dist += dirn.getPolyline().Distance();
      document.getElementById("distance").innerHTML="Path length: "+(dist/1000).toFixed(2)+" km. "+(dist/1609.344).toFixed(2)+" miles.";
    }
    firstpoint = false;
  });

  GEvent.addListener(dirn,"error", function() {
    GLog.write("Failed: "+dirn.getStatus().code);
  });

}
else {
  alert("Sorry, the Google Maps API is not compatible with this browser");
}

// This Javascript is based on code provided by the
// Community Church Javascript Team
// http://www.bisphamchurch.org.uk/   
// http://econym.org.uk/gmap/

//]]>
</script> 

You can click and add paths that trace routes.

I would like to use the trace-road-functionality, but pre-define the data using coordinates instead. How would I do this?