用法
改用！
var container = document . querySelector ( '#myContainer' )
var options = {
responsive : true ,
fullscreen : false
}
// a canvas will be appended to container
// if container is left unspecified the default container is document.body
// the width and height of canvas is equal to container
var ctx = get2dContext ( container , options )
if ( ! ctx ) {
// canvas is not supported
