jointjs vertice don't drag correctly on mobile -

vertices don't move correctly (drag) on mobile phone whereas it's ok on pc.
simple example bug can observed.

var graph = new joint.dia.graph; var paper = new joint.dia.paper({ el: $('#easel'), width: 600, height: 200, gridsize: 1, model: graph });  var parent = new joint.shapes.basic.rect({ position: {x: 100,y: 30}, size: {width: 100,height: 30}, attrs: {rect: {fill: 'blue'},         text: {text: 'parent',fill: 'white'}} }); var child = new joint.shapes.basic.rect({ position: {x: 70,y: 130}, size: {width: 70,height: 30}, attrs: {rect: {fill: 'lightgreen',rx: 5,ry: 5},         text: {text: 'child',fill: 'white'}} });  var link = new joint.dia.link({ source: {id: parent.id}, target: {id: child.id}, vertices: [{x: 210,y: 75}, {x: 190,y: 105}] });  graph.addcells([parent, child, link]); 

does have idea?