Change value of gage dynamically

I am developing a fitness app in which i want to represent BMI through gage but didn't getting how to change the value of gage dynamically as well second problem is that when i am putting gage script on button click it giving me error "No element with id: "gauge" found!". Can anyone solve my both issues as i am new to JS. So enlighten me and correct me.

<html>
<head>
<script src="raphael.2.1.0.min.js"></script>
<script src="justgage.1.0.1.min.js"></script>

<script type="text/javascript">
function funky()
{
    var weight=document.getElementById("weight");
    var height=document.getElementById("height");
    var heightDouble=height.value*height.value;
    var bmi=weight.value/heightDouble;
    document.getElementById("bmi").innerHTML=bmi;
    cc();
}
</script>
</head>
<body>
<input type="text" id="weight"/>
<input type="text" id="height"/>
<input type="button" id="calculate" onClick="funky()"/>
<div id="bmi"></id>
<div id="gauge"></div>

           <script>
           function cc()
           {               
var g = new JustGage({
id: "gauge",        
value: 10,
min: 0,
max: 100,
title: "Visitors"
}); 
}   
</script>

</body>
</html>

Solution

You will want to fix your BMI calculation and add units (lbs vs. kg, cm vs. in), because I am getting numbers below zero. At any rate, I wrote a new calculation (just add weight and height) to demonstrate how to update the gauge: Most likely you are getting the "gauge not found" error because of the way the script is ordered relative to the div element.

Please see This JSFiddle demo

<script src="//cdn.jsdelivr.net/raphael/2.1.2/raphael-min.js"></script>
<script src="//cdn.jsdelivr.net/justgage/1.0.1/justgage.min.js"></script>
<body>
    <input type="text" id="weight" />
    <input type="text" id="height" />
    <input type="button" id="calculate" onClick="funky()" value="calculate" />
    <div id="bmi"></div>
    <div id="gauge" class="200x160px"></div>
    <script>
        function funky() {
            var weight = document.getElementById("weight");
            var height = document.getElementById("height");
            var heightDouble = height.value * height.value;
            var bmi = weight.value / heightDouble;
            document.getElementById("bmi").innerHTML = bmi;
            //cc();
            var weightPlusHeight = parseInt(weight.value) + parseInt(height.value);
            g.refresh(weightPlusHeight);
        }

        var g = new JustGage({
            id: "gauge",
            value: 0,
            min: 0,
            max: 200,
            title: "BMI"
        });
    </script>
</body>