Edit in JSFiddle

<button>+</button>
<button>-</button>

<div id="graph"></div>
// Custom Arc Attribute, position x&y, value portion of total, total value, Radius
var archtype = Raphael('graph', 100, 100);

// Manages creating the Path string based on angle of the arc
archtype.customAttributes.arc = function (value) {
    var xloc = 50,
        yloc = 50,
        total = 100, 
        R = 30,
        alpha = 360 / total * value,
        a = (90 - alpha) * Math.PI / 180,
        x = xloc + R * Math.cos(a),
        y = yloc - R * Math.sin(a),
        path;
    if (total == value) {
        path = [
            ["M", xloc, yloc - R],
            ["A", R, R, 0, 1, 1, xloc - 0.01, yloc - R]
        ];
    } else {
        path = [
            ["M", xloc, yloc - R],
            ["A", R, R, 0, +(alpha > 180), 1, x, y]
        ];
    }
    
    return {
        path: path
    };
};

//make an arc at 50,50 with a radius of 30 that grows from 0 to 40 of 100 with a bounce
var my_arc = archtype.path().attr({
    "stroke": "#0f0",
    "stroke-width": 14,
    arc: 0
});

var my_text = archtype.text().attr({
    x : 50,
    y : 50,
    text: '0%'
});

function setAngle (x) {
    var color = '#0f0';
    
    if (x > 33) {
        color = '#FF0';        
    }
    if (x > 66) {
        color = '#F00';
    }
    
    my_text.attr({
        text: '' + x + '%'
    });
    
    my_arc.animate({
        "stroke": color,
        arc: x
    }, 1000, "bounce");
}

var x = 30;

setAngle(x);

$('button').on('click', function (e) {
    if ($(e.target).html() === '+') {
        x = Math.min(100, x + 10);
    } else {
        x = Math.max(0, x - 10);
    }
    
    setAngle(x);
})

External resources loaded into this fiddle: