var scale_factory_cache = {};

function scale_factory(feature) {
    var dim = Math.round(+feature.properties.Burglary / 100);

    if (!scale_factory_cache[dim]) {
        var c = document.createElement('canvas');
        c.width = dim;
        c.height = dim;
        var ctx = c.getContext('2d');
        ctx.fillStyle = 'rgba(142, 179, 219, 0.5)';
        ctx.strokeStyle = '#3F71A7';
        ctx.beginPath();
        ctx.arc(dim/2, dim/2, dim/Math.PI, 0, Math.PI*2, true);
        ctx.closePath();
        ctx.fill();
        ctx.stroke();
        scale_factory_cache[dim] = c.toDataURL();
    }

    var el = document.createElement('img');
    el.width = dim;
    el.height = dim;
    el.src = scale_factory_cache[dim];
    el.style.cssText =
        'width:' + dim + 'px;' +
        'height:' + dim + 'px;' +
        'margin-left:' + (-dim/2) + 'px;' +
        'margin-top:' + (-dim/2) + 'px;' +
        'position:absolute';
    return el;
}