The documentation says:

The alignment determines where the label is drawn relative to the marker's position.

Parameters:
align Alignment. A combination of AlignTop, AlignBottom, AlignLeft, AlignRight, AlignCenter, AlgnHCenter, AlignVCenter.

See also:
labelAlignment()

Definition at line 312 of file qwt_plot_marker.cpp.

What I wanted was centered relative to the plot boundaries, not relative to the marker position.