These will be useful if one wants to put additional things on the bars.

Noted in#30224 (comment), which this will simplify because instad of

defcenter_top(rect):returnrect.xy[0]+rect.get_width()/2,rect.xy[1]+rect.get_height()xy=np.array([center_top(r)forrincontainer.patches])ax.errorbar(xy[:,0],xy[:,1],yerr=yerr)

we can then simply do

ax.errorbar(container.position_centers,container.tops,yerr=yerr)

Naming: The naming has to work for horizontal and vertical bars.

• position_centers: I useposition as the non-data coordinate. This is consistent with the naming inhttps://matplotlib.org/devdocs/api/_as_gen/matplotlib.axes.Axes.grouped_bar.html (bar()/barh() usex/y, which doesn't generalize to orientation.
  I intentionally choseposition_centers overcenter_positions, becausecenter_position could be misinterpreted as the (x, y) coordinates of the centers of the bar rectangles.
• tops:https://matplotlib.org/devdocs/api/_as_gen/matplotlib.axes.Axes.grouped_bar.html universally usesheights and the upper end of theheight is reasonably thetop. I think its bearable to have this naming also in the face of potentially horizontal bars.