public void AddEllipse(double rx, double ry, double cx, double cy)
 {
     cx = _calcx(cx);
     cy = _calcy(cy);
     rx = _scale(rx);
     ry = _scale(ry);
     _svgWriter.AddEllipse(rx.ToString("0.##"), ry.ToString("0.##"), cx.ToString("0.##"), cy.ToString("0.##"));
 }