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.');
}