circle.skeleton

var plot = new Plotter("plot", {
    left: -3,
    right: +3,
    bottom: -3,
    top: 3,
    width: 600,
    height: 600
});

var borders = {
    left: -1,
    right: 1
};

plot.addFunc(function (x) {
    return Math.sqrt(1 - x*x);
}, borders);

plot.addFunc(function (x) {
    return -Math.sqrt(1 - x*x);
}, borders);