const double r = 300;
var func1 = new Func<double, double?>((x) => Math.Sqrt(Math.Pow(r, 2) - Math.Pow(x, 2)));
var func2 = new Func<double, double?>((x) => -Math.Sqrt(Math.Pow(r, 2) - Math.Pow(x, 2)));

formsPlot1.Plot.Clear();

// Add functions to the plot
formsPlot1.Plot.AddFunction(func1, lineWidth: 3, color: Color.Blue);
formsPlot1.Plot.AddFunction(func2, lineWidth: 3, color: Color.Blue);

// Manually set axis limits because functions do not have discrete data points
formsPlot1.Plot.SetAxisLimits(-300, 300, -300, 300);
               
formsPlot1.Plot.Render();
formsPlot1.Refresh();