var chart = new d3Kit.CanvasChart('#chart', {  
  initialWidth: 720,
  initialHeight: 500,
  margin: { top: 30, right: 30, bottom: 30, left: 30 }
});

var ctx = chart.getContext2d();
ctx.fillStyle = 'red';
ctx.fillRect(10, 10, chart.getInnerWidth(), chart.getInnerHeight());