Plotting certain columns after using gruopby

Question

I have the following code:

data = {\'Customer_id\': [1, 2, 1, 2, 1, 2, 3, 4, 4, 1, 2, 1, 2, 3, 4, 4, 2, 1, 3],
\'order_time\': [\'2010-01-01 01:01:00\', \'2