How to add a line at x=y to a contour plot?

Question

I cant figure out how to add a line to my contour plot at x=y.

epsilon_1= np.linspace(1,25,1000)
epsilon_2= np.linspace(1,25,1000)
eps1, eps2 = np.meshgrid( epsilon_1, epsilon_2 )
plt.contourf( eps1, eps2, E, 100 ,cmap="jet")

is what I have and I'm not sure how to add this line if its even possible?

I wasn't sure what to try.