code: function (howMuch, centerXArg, centerYArg) {
  
  			if (this.isMapZooming) {
  				return;
  			}
  
  			var chart = this,
  				xAxis = chart.xAxis[0],
  				xRange = xAxis.max - xAxis.min,
  				centerX = pick(centerXArg, xAxis.min + xRange / 2),
  				newXRange = xRange * howMuch,
  				yAxis = chart.yAxis[0],
  				yRange = yAxis.max - yAxis.min,
  				centerY = pick(centerYArg, yAxis.min + yRange / 2),
  				newYRange = yRange * howMuch,
  				newXMin = centerX - newXRange / 2,
  				newYMin = centerY - newYRange / 2,
  				animation = pick(chart.options.chart.animation, true),
  				delay,
  				newExt = chart.fitToBox({
  					x: newXMin,
  					y: newYMin,
  					width: newXRange,
  					height: newYRange
  				}, {
  					x: xAxis.dataMin,
  					y: yAxis.dataMin,
  					width: xAxis.dataMax - xAxis.dataMin,
  					height: yAxis.dataMax - yAxis.dataMin
  				});
  
  			xAxis.setExtremes(newExt.x, newExt.x + newExt.width, false);
  			yAxis.setExtremes(newExt.y, newExt.y + newExt.height, false);
  
  			// Prevent zooming until this one is finished animating
  			delay = animation ? animation.duration || 500 : 0;
  			if (delay) {
  				chart.isMapZooming = true;
  				setTimeout(function () {
  					chart.isMapZooming = false;
  				}, delay);
  			}
  
  			chart.redraw();
  		}
docs: Zoom the map in or out by a certain amount. Less than 1 zooms in, greater than 1 zooms out.