How to add regline to df.plot.scatter

I've done df.plot.scatter like this

df = pd.DataFrame({'x':y_pred, 'y':y_test})
df.plot('x', 'y', kind='scatter', s=50)

I want to add regline & i have no idea how to do this

of course I can do by sns.lmplot but i like this style

python 3