Bugs for transformLimits?

Such as example below. I want to put a new ax at data coordinates [(45,45),(50,50)], but the transform fail to transform data coordinates to axes.

import numpy as np

import pandas as pd

import matplotlib.pyplot as plt

import matplotlib as mpl

from matplotlib.backends.backend_pdf import PdfPages

import matplotlib.transforms as mtrans

%matplotlib inline

fig = plt.figure()

ax = fig.add_subplot(121,aspect=0.5)

ax.plot(np.linspace(0,100))

ax.set_ylim(0,50)

pbcb = mtrans.Bbox(ax.transLimits.transform([(45,45),(50,50)]))

cbax = fig.add_axes(pbcb)