import matplotlib
import matplotlib.pyplot as plt
from matplotlib.transforms import Bbox, TransformedBbox

fig, ax = plt.subplots()

rect = ax.axvspan(0.5, 0.55, 0, 1)

transform = ax.get_xaxis_transform()

bbox = Bbox.from_extents(0.5, 0, 0.55, 1)
tbbox = TransformedBbox(bbox, transform)

text_kw= dict()
text_kw.setdefault("va", "top")
text_kw.setdefault("clip_on", True)
text_kw.setdefault("bbox", dict(boxstyle="round", facecolor="white"))


ann = ax.annotate(
    "Annotation",
    xy=(0.5, 1),
    xytext=(8, -10),
    textcoords="offset points",
    clip_path=rect,
    clip_box=tbbox,
    **text_kw,
)

ax.set(xlim=[0, 1])