Changing Ellipse Radius in C#

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C#

Windows

WinForms

C#4.0

VS2010

, +

I have made a Windows Forms Application using C# that displays on the screen ellipse which has color filled according to pressure values of a sensor. This ellipse represents a region of the foot.
I want to change the radius of the ellipse with the increase in input, such that for a low pressure value the ellipse has a small radius and as input increases the ellipse should keep getting bigger.
I tried increasing its width and height but that would make its center shift.
Is there any other way in which I can achieve this, and also not change the center of my ellipse??

Heres a part of the code that i have used.

{
                    //Lies within a for loop

                    path.AddEllipse(ebound_left_mid1);
                    //increase width and height
                    ebound_left_mid1.Width += 1;
                    ebound_left_mid1.Height += 1;
                    // ebound_left_mid1
                    PathGradientBrush br1 = new PathGradientBrush(path);
                    br1.CenterColor = clr_arry[1];
                    br1.SurroundColors = new Color[] { Color.White};




                    surface.DrawImage(myImg, DrawingRectangle);
                    surface.FillEllipse(br1, ebound_left_mid1);

                    buff.Render();
                }