Category "standard-deviation"

How can I draw a line in a python plot indicating 1 standard deviation?

I have a plot from a data frame, I want to to draw a line indicating one standard deviation from the mean like in the picture below. I know how to add text in

Standard Deviation from multiple files in bash

I wish to calculate the standard deviation from a range of files titled "res_NUMBER.cs" which are formatted as a CSV. Example data includes 1,M,CA,54.9130 1,M