[Python] How do I add an x=y line through this seaborn plot?

I have a logarithmic scatter plot in seaborn, but I am wondering how to put an x=y line to bisect the data

ax = sns.pairplot(x_vars=["Yamaguchi Double"], y_vars=["Yamaguchi Helix"], data=df11,
hue="Image", size=3)
ax.set(xscale="log", yscale="log", xlim=(1e-3,1e1), ylim=(1e-3,1e1))


This produces this image:

logarithmic plot

How do I add a red diagonal x=y line over this?

I've tried this code:

ax = sns.pairplot(x_vars=["Yamaguchi Double"], y_vars=["Yamaguchi Helix"], data=df11,
hue="Image", size=3)
ax.set(xscale="log", yscale="log", xlim=(1e-3,1e1), ylim=(1e-3,1e1))
X_plot = np.linspace(0.001, 10)
Y_plot = X_plot
plt.plot(x="X_plot", y="Y_plot", color = 'r')
plt.show()


but this gave me no line

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