The drawing an elipse with four particular points on the x/y axis.

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The drawing an elipse with four particular points on the x/y axis.


I've been using this function but have noticed that if you position say your first and second points below the horizontal x axis and then add the other two points when the ellipse is drawn the first two have moved. They don't stay locked exactly in their original locations. For instance if you draw a horizontal line 300mm long at say at -20mm on the y axis then draw the following ellipse using the four sets of coordinates 1st point -100,-20  2nd point 100,-20 3rd point 0,120 and the fourth point at 0,-120 you'll notice that the horizontal line at -20mm doesn't pass through the widest point of the ellipse. The widest point of the ellipse is actually several mm above the horizontal line drawn at what should be the y axis coordinate of first two selected points. The reason I'm trying to get this right is because I'm using it to draw sectionals for an aircraft fuselage where as they move forward towards the nose cone and get smaller the horizontal center line of the fuselage is not the same as the widest part of the aircraft body which actually moves down towards the nose.

Any thoughts as to why these points don't stay locked but seem to move up on the y axis when the ellipse is completed?

Many thanks