See this at jsfiddle.net/canvasjs/QwZuf
function ChangeScale() {
	var iMax = chart.axisY[0].maximum;
  var iMaxNew = iMax * 3;
  var iInterval = chart.axisY[0].interval;
  var iIntervalNew = iInterval * 3;
  
  chart.axisY[0].maximum = iMaxNew;
  chart.axisY[0].interval = iIntervalNew;
  chart.render();
  
  alert(iMaxNew + ' is bigger than ' + iMax + ', but the scale did not change.');
}