Fix cases where no plot is generated and add tests to validate saving of plots

[szaydel]

Hi! Please review this changeset carefully and confirm that it is still in line with project goals. Mainly, I focused on adding some basic tests for validating plot generation. In my tests checksums were consistently as expected, but I am not positive that I can expect this to always be true. I am also not sure if TravisCI can run tests where temp files are generated and checksums are obtained by reading those files. This needs further testing.

Perhaps it might be good to move these into their own branch and I would not mind fetching that branch and testing with it. It is possible that because automatic merge won't be done, that a merge conflict needs further attention.

Thank you, Sam.