ax.annotate(label, xy=(r, 0), xycoords=tform, xytext=(r, 0.9), textcoords=
    tform, fontsize='xx-large', ha='center', va='center', color='r',
    arrowprops=arrowprops)
Is it possible to anchor a matplotlib annotation to a data coordinate in the x-axis, but to a relative location in the y-axis?