Hi Cho,
In order to revert to the original zooming, you need to rest the content area zoom factor:
NPolarChart polarChart = nChartControl1.Charts[0] as NPolarChart;
polarChart.ContentAreaZoomFactor = new NRectangleF(0, 0, 1, 1);
nChartControl1.Refresh();
Let us know if you meet any problems.
Best Regards,
Nevron Support Team