how to set the y reference points matplotlib code example

Example: show graph matplotlib axes

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

df = pd.read_csv("filesc/file1.csv")
df.head()

BBox = ((df.x.min(),   df.x.max(), df.y.min(), df.y.max()))

ruh_m = plt.imread('map.png')

print(BBox)

fig, ax = plt.subplots(figsize = (8,7))
ax.scatter(df.x, df.y, zorder=1, alpha= 0.2, c='b', s=10)
ax.set_title('Plotting Spatial Data on Map')
ax.set_xlim(BBox[0],BBox[1])
ax.set_ylim(BBox[2],BBox[3])
ax.imshow(ruh_m, zorder=0, extent = BBox, aspect= 'equal')
plt.show()