[Google Maps API v2] Heat Maps Generation error

Hi
I have generated the Google maps successfully.When I click on the
link,Google maps are generation. In this i am also using MarkerIcons
also to generate the data in different colors.Now i want to generate
the heat maps for the same database.

for that the steps i followed are:
After generating of Google maps I called the initheatmap() in the
place of (map.addOverlay(markers)) I have commented this line

Then this will call the initheatmap and have written these steps also:

function initHeatmap()
{
var myHM = new GEOHeatmap();
myHM.Init(800, 700);

addMarkersFromXML();
document.getElementById("HMMapdata").value = batch;

myHM.SetBoost(1);
myHM.SetDecay(0);

//salert(batch);
var preUrl = myHM.GetURL();
alert(preUrl);
var heatmapOverlay = new HMGoogleOverlay(preUrl);
map.addOverlay(heatmapOverlay);
}

function addMarkersFromXML()
{
var request=GXmlHttp.create();
request.open('get','path to xml file ',true);
request.onreadystatechange=function()
{
if(request.readystate==4)
{
var xmlDoc=request.responsexml;
var
xxx=xmlDoc.documentElement.getElementsByTagName("Markers");
for (var i = 0; i <Markers.length; i++) {
some code to call the data

}

}
}
request.send(null);
}

I am not getting any error but heat maps are not being generated.

can u suggest me the solution where i went wrong.

Thanks

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