如何从关节下载图形

如何解决如何从关节下载图形

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(255,165,0.3)'
    }
});

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)

如何下​​载PNG，SVG或JPEG格式的图形？