var viewer = new Cesium.Viewer('cesiumContainer');

var purpleArrow = viewer.entities.add({
    polyline : {
        positions : Cesium.Cartesian3.fromDegreesArrayHeights([-75, 43, 500000,
                                                               -80, 43, 0,
                                                               -85, 43, 500000]),
        width : 10,
        followSurface : false,
        material : Cesium.Color.PURPLE
    }
});

viewer.zoomTo(viewer.entities);