<aura:application >
    <canvas aura:id="chart" />
</aura:application>

({
    // Your renderer method overrides go here
    afterRender: function(component, helper) {
        var canvas = component.find("chart").getElement();
        var context = canvas.getContext("2d");
        context.fillStyle="black";
        context.arc(50, 50, 50, 0, 2*Math.PI);
        context.fill();
    }
})