function getNumberImageUrl(num){
    return "http://chart.apis.google.com/chart?chxt=x&chxp=0,0&chs=50x20&cht=ls&chd=t:60&chxs=0,999999,17&chxl=0:|"+num;
}

function updateCharCount(textAreaId) {
    var area = document.getElementById(textAreaId);
    var text = area.value ? area.value : "";
    url = getNumberImageUrl(140-text.length);
    setBackgroundImage(textAreaId, url);    
}

function setBackgroundImage(elemId, imageUrl){
    document.getElementById(elemId).style.backgroundImage = "url("+imageUrl+")";
}