Javascript 如何从关节下载图形

如何下载PNG、SVG或JPEG格式的图形形状

var graph = new joint.dia.Graph;

var paper = new joint.dia.Paper({
    el: document.getElementById('myholder'),
    model: graph,
    width: 800,
    height: 500,
    gridSize: 20,
    drawGrid:true,
    background:{
      color:'rgba(255,165,0,0.3)'
    }
});

var rect = new joint.shapes.standard.Rectangle();
rect.position(100, 30);
rect.resize(100, 40);
rect.attr({
    body: {
        fill: 'green'
    },
    label: {
        text: 'Activity1',
        fill: 'white'
    }
});

rect.addTo(graph)