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Circle shaper centered in a Ternary Plot

I have a basic ternary plot.
I need to center in my plot a circle shape as the image:

Right now, the options for the position of the shape are:
type: ‘circle’,
xsizemode: ‘scaled’,
ysizemode: ‘scaled’,
xref: ‘paper’,
x0: 0.40,
x1: 0.60,
yref: ‘paper’,
y0: 0.21,
y1: 0.49

But this shape is not perfectly centered, and also I’ve problems with the resizing of the page (the circle is deforming).

Anyone can help me please?