Streamline xanchor and yanchor names for center/middle

The 'xanchor' property is an enumeration that may be specified as:
- One of the following enumeration values:
['auto', 'left', 'center', 'right']

The 'yanchor' property is an enumeration that may be specified as:
- One of the following enumeration values:
['auto', 'top', 'middle', 'bottom']

This is for plotly 6.3.0 trying to use fig.add_annotations() but it likely applies everywhere where xanchor and yanchor are used.

It does make sense that for the extremes of the bounding box left right top bottom, the names are different, but why is 'center' used for x direction and 'middle' for y direction? This distinction seems artifical to me and I wonder why this was chosen?

I propose to streamline the naming conventions, but now for backwards compatibility to at least accept both center/middle for x/yanchor.

Commencez par le point d’entrée fig.add_annotations() et identifiez comment les valeurs de xanchor et yanchor sont validées. Vérifiez si la même validation est partagée ailleurs, puis examinez les tests associés. La réalisation doit inclure, le cas échéant, l’acceptation rétrocompatible de center et de middle, tout en conservant le fonctionnement des valeurs existantes.