Update example code in docs to better showcase Plot class

[tlisonbee]

The Plot class gives a layer of abstraction over the underlying Data and Assign classes but most of the docs show the lower level interfaces. It would be nice if the example code in the docs showed both styles better.

[tlisonbee]

E.g. Some example code like is found here https://github.com/Nike-Inc/signal_analog/blob/master/examples/full_data_pipeline_cloudwatch_example.py could be added to the README