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I'm running into some problems plotting 2D Ellipsoids with non-diagonal weight matrices -- they are not centered where they are supposed to be. I'm not sure exactly what the problem is, it only seems to happen when the numbers are $\gg1$. The ellipse can even change positions if you change the plot range. The same thing happens with both Epilog and Show.

I am using version 10.0.2.0. Thanks for any help or workarounds. I am trying to plot ellipses corresponding to confidence regions on top of density plots, so the scales of the axes matter to me.

Row[
    Plot[x, {x, -500, 500}, 
        Epilog -> {Opacity[0], EdgeForm[Thick], 
            Ellipsoid[{0, 0}, 
                RotationMatrix[#].{{40000, 0}, {0, 20000}}.RotationMatrix[-#]]}, 
        AspectRatio -> 1, ImageSize -> 300
    ] & /@ {0, 10^-8, π/4}
]

enter image description here

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1 Answer 1

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The second argument of Ellipse seems to need to be a vector (and not a matrix). In any case, try rotating the object. You'll also want to change the position ({0,0} below) to get it centered where you want. Something like:

Row[Plot[x, {x, -500, 500}, 
    Epilog -> {Opacity[0], EdgeForm[Thick], 
      Rotate[Ellipsoid[{0, 0}, {400, 200}], #]}, AspectRatio -> 1, 
    ImageSize -> 300] & /@ {0, \[Pi]/4}]

enter image description here

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  • $\begingroup$ Thanks, good fix. "The second argument of Ellipse seems to need to be a vector" -- the docs are clear it can be a matrix, I guess you mean it needs to be a vector to plot properly. They must have different plotting code for each case, and they also check to see if the matrix is diagonal or not. $\endgroup$
    – Ian Hincks
    Commented May 12, 2016 at 16:04
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    $\begingroup$ I think there is may be some kind of bug. Consider Plot[x, {x, -500, 500}, Epilog -> {Rotate[ Ellipsoid[{0, 0}, {{4000, 0}, {0, 2000}}], \[Pi]/4]}, AspectRatio -> 1] When I plot this, and move the cursor up and down, the position of the ellipse seems to jump around. $\endgroup$
    – bill s
    Commented May 12, 2016 at 16:11
  • $\begingroup$ I have noticed weird stuff like this too! I was just not sure how to explain how to reproduce it, or if it was just another of the many small GUI problems with Mathematica in linux. $\endgroup$
    – Ian Hincks
    Commented May 12, 2016 at 16:14
  • $\begingroup$ To make your answer more general (in most practical cases the rotation matrix would not be explicitly there), something like makeEllipse[p_, mat_] := Module[{d, v}, {d, v} = Eigensystem[mat]; Rotate[Ellipsoid[p, Sqrt@d], ArcTan @@ (First@v)]] would be good. Thanks again, I will accept. $\endgroup$
    – Ian Hincks
    Commented May 12, 2016 at 16:15

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