seaborn add line

g=sns.relplot(x="Ground CCN $(cm^{-3})$", y="VIIRS CCN $(cm^{-3})$", hue="Stid", size="SS",
            sizes=(40, 200), alpha=.5, palette="muted",
            height=6, data=df)
#g.map(plt.axhline,y=400,color="k", ls=":")
#plt.axhline(400)
plt.xlim(0,4500)
plt.ylim(0,4500)
plt.plot(np.arange(4000)*0.5)
plt.savefig("ccn_compare112.png",dpi=600)

https://jakevdp.github.io/PythonDataScienceHandbook/04.14-visualization-with-seaborn.html

https://matplotlib.org/3.1.1/gallery/text_labels_and_annotations/multiline.html#sphx-glr-gallery-text-labels-and-annotations-multiline-py