test plot extraction routines and document the useful ones

[petermr]

The package org.contentmine.graphics.svg.plot contains many plot extraction tests. Run them, list failures and errors in Issues and document briefly those useful for data extraction. @OmarBani

[petermr]

The package org.contentmine.graphics.svg.plot contains many tests for extracting data. Some are long, some are tied to PMR-local data files. None are usefully documented.

• run the tests and determine the cause of failure. In some cases the data file will be missing and can be found in the cephis package. Copy these over and rerun. In other cases the extracted values won't match the test - PMR should inspect these.
• write a brief Javadoc description for the useful tests. This should include whether it tests a specific component, e.g. axis, error bars, etc.
• create a README.md for the plot package, including the output of the javadoc.

Good luck. PMR will be online and has adequately functioning computers.