justGage - increase and decrease value with buttons

I'm using justGage (a plugin for generating and animating nice & clean dashboard gauges) to program some graphs - I need to be able to increase and decrease the value by clicking a button.

I can do this by using refresh (g1.refresh(20);) - but that makes the value 20...it doesn't increase it by 20.

Can anyone help please ?

Many thanks Al

I couldn't find a method for retrieving the gauge's current value, so I just store that value in a separate variable. Then, every time the gauge is updated, this variable is too.

Demo

<script src="http://cdn.jsdelivr.net/raphael/2.1.2/raphael-min.js"></script>
<script src="http://cdn.jsdelivr.net/justgage/1.0.1/justgage.min.js"></script>

    <input type="button" id="inc" onClick="inc()" value="increase by 20" />
    <input type="button" id="dec" onClick="dec()" value="decrease by 20" />
<br><br>
    amount to change: <input type="text" id="change" />
<br>
    <input type="button" id="incVar" onClick="incVar()" value="increase (variable)" />
    <input type="button" id="decVar" onClick="decVar()" value="decrese (variable)" />

    <div id="gauge" class="200x160px"></div>
    <script>

var gageValue = 100;

        var g = new JustGage({
            id: "gauge",
            value: gageValue,
            min: 0,
            max: 200,
            title: "BMI"
        });

function updateGage(n) {
  g.refresh(gageValue + n);
  gageValue += n;
}

function inc() {
  updateGage(20);
}

function dec() {
  updateGage(-20);
}

function incVar() {
  updateGage(parseInt(document.getElementById("change").value))
}

function decVar() {
  updateGage(-parseInt(document.getElementById("change").value))
}

</script>