It's easiest to use the ChartSymbol class, which already has a CIRCLE type defined. The SymbolSize is specified in pixels, the location in the current plot scale.
ChartAttribute attrib4 = new ChartAttribute(Color.Red, 1, DashStyle.Solid);
attrib4.SymbolSize = 64;
ChartSymbol circle = new ChartSymbol(pTransform1, ChartObj.CIRCLE, attrib4);
circle.SetLocation(-10, 20);
chartVu.AddChartObject(circle);