Hi Manal,
There is no way to achieve this functionality using the NChartControl control. This feature is available only for the NThinWebChartControl. Is there any reason why you want to use the NChartControl? - as the NThinWebChartControl offers the same charting functionality. Regarding the zooming - you need to disable the Y axis zooming from the NDataZoomTool AllowYAxisZoom property:
NDataZoomTool dataZoomTool = new NDataZoomTool();
dataZoomTool.AllowYAxisZoom = false;
NThinChartControl1.Controller.Tools.Add(dataZoomTool);
Hope this helps - let us know if you meet any problems or have any questions.