how to get the sum of distances from the diagonal line in a scatter plot in python?

Question

I have a pandas dataframe

import pandas as pd
foo = pd.DataFrame({\'col_a\': [1,2,3,4,5], \'col_b\': [1,3,2,5,4], \'group\': [\'a\',\'a\',\'a\', \'b\',\'b\']})