This is a follow-up question to

Adding markers or lines to colorbar in matplotlib

I would like to indicate positions on a colorbar that extends over the length/width of the colorbar itself.

I tried matplotlib.pyplot.hlines as well as matplotlib.axes.Axes.axhline and simply adding a line using matplotlib.axes.Axes.plot adjusting the xmin and xmax-values but nothing let me extend over the boundaries of the colorbar.