Trim data on matplotlib diagonally

Question

Can anyone help me with how to trim plot data on the left side of this straight line besides doing it manually on excel? Thanks

enter image description here

I'm using pandas to load and read the excel data, this is my code:

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
import matplotlib.cm as cm
plt.style.use('seaborn-poster')
import seaborn as sns

fig, ax = plt.subplots(1,1)
x = df['Ex Wavelength(nm)']
y = df['Em Wavelength(nm)']
Z = df['Intensity(cnt)']

ax.tricontourf(x, y, Z, cmap=plt.cm.viridis)
ax.set_xlabel('Excitation (nm)', labelpad=20)
ax.set_ylabel('Emission (nm)', labelpad=20)

x1 = [200, 300, 450, 200]
y1 = [500, 500, 750, 750]
mask = ax.add_patch(patches.Polygon(xy=list(zip(x1,y1)), fill=True))

m = cm.ScalarMappable(cmap=cm.viridis)
m.set_array(df['Intensity(cnt)'])
plt.colorbar(m, pad=0.15)

plt.show()

enter image description here

I use this excel file: https://docs.google.com/spreadsheets/d/1PuGE6Jfx_ZG2yfczJxHL1H88OnK9d5WU/edit?usp=sharing&ouid=116799328907524097274&rtpof=true&sd=true