Fix 3D ellipsoid scaling around its center

[FlorianPfaff]

Bug

plot_ellipsoid_3d() applied scaling_factor to the already translated surface coordinates. For an ellipsoid centered away from the origin, changing its scale therefore moved the center as well as resizing the axes.

For example, an identity ellipsoid centered at [3, 4, 5] with scaling_factor=2 produced a vertical extent from 8 to 12, centered at 10, instead of the correct extent from 3 to 7, centered at 5.

Root cause

The helper first added center to the unit-sphere transform and then multiplied x, y, and z by scaling_factor in plot_surface(). Algebraically, this computed scale * (shape @ point + center) rather than center + scale * shape @ point.

Fix

• apply scaling_factor only to the transformed ellipsoid offsets;
• translate by center afterward;
• pass the final world coordinates directly to Matplotlib;
• add a regression using a non-origin center and a scale factor of two.

Validation

• the regression fails against the previous implementation with z=12 where z=7 is expected;
• the focused regression passes with the patch;
• syntax compilation passes for both the modified implementation and the new test;
• branch is 2 commits ahead and 0 behind main, with changes limited to the plotting helper and one regression test.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	23.54s
✅ JSON	prettier	7	0	0	0	1.45s
✅ JSON	v8r	7		0	0	3.84s
✅ MARKDOWN	markdownlint	68	0	0	0	1.86s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.9s
✅ PYTHON	black	1810	56	0	0	84.56s
✅ PYTHON	isort	1810	88	0	0	2.74s
✅ REPOSITORY	betterleaks	yes		no	no	1.57s
✅ REPOSITORY	checkov	yes		no	no	52.52s
✅ REPOSITORY	gitleaks	yes		no	no	15.66s
✅ REPOSITORY	git_diff	yes		no	no	0.11s
✅ REPOSITORY	secretlint	yes		no	no	78.68s
✅ REPOSITORY	syft	yes		no	no	5.37s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.32s
✅ REPOSITORY	trufflehog	yes		no	no	31.84s
✅ YAML	prettier	11	0	0	0	0.79s
✅ YAML	v8r	11		0	0	12.04s
✅ YAML	yamllint	11		0	0	0.53s

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