Hi Massimiliano,
You can specify min axis dock zone size - for example the following code sets the minimum left axis dock zone size to 50 points:
NCartesianChart chart = (NCartesianChart)nChartControl1.Charts[0];
chart.MinDockZoneMargins = new NMarginsL(50, 0, 0, 0);
That way if you provide a sufficiently large number the axis size will not change when the data changes. Hope this helps - let us know if you meet any problems or have any questions.