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Numerical instability:
\verb|\pgfmathparse{acos(-1.00008* 0.99992)}\pgfmathresult|
\pgfmathparse{acos(-1.00008* 0.99992)}\pgfmathresult
; should be 180 --- three degrees difference. pgfpatharc uses these angles and causes thus visible errors.  work--around: don't use the dots notation in xtick/ytick

Fixed now: I implemented a specialized patharc

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