Matplotlib – Adding caption below X-axis for a scatter plot using matplotlib

I am pretty new to python and to the matplotlib library. I have created a scatter plot using matplotlib and now I wish to add caption a little below the X-axis. This is my code:

from matplotlib import pyplot as plt
import numpy as np
from pylab import *

file = open('distribution.txt', 'r')

txt="I need the caption to be present a little below X-axis"

x=[]
y=[]
for line in file:
    new=line.rstrip()
    mystring=new.split("\t")
    x.append(mystring[0])
    y.append(mystring[1])


fig = plt.figure()
ax1 = fig.add_axes((0.1,0.4,0.8,0.5))
ax1.set_title("This is my title")
ax1.set_xlabel('X-axis')
ax1.set_ylabel('Y-axis')
ax1.scatter(x,y, c='r')
fig.text(.05,.05,txt)
plt.xlim(0, 1.05)
plt.ylim(0, 2.5)
plt.show()

As you can see in the image my caption is way below the scatter plot, is there a way to bring it exactly below the X-axis? Also my scatter plot looks rectangular, is there a way to make it square like?