graph.addCell(cell)

Add a new cell to the graph. If cell is an array, all the cells in the array will be added to the graph.

var rect = new joint.shapes.basic.Rect({ 
    position: { x: 100, y: 100 },
    size: { width: 70, height: 30 },
    attrs: { text: { text: 'my rectangle' } }
})
var rect2 = rect.clone()
var link = new joint.dia.Link({ source: { id: rect.id }, target: { id: rect2.id }  });
var graph = new joint.dia.Graph
graph.addCell(rect).addCell(rect2).addCell(link)