Plotting utils

We really need some simple functions for manipulating data. These include subtracting a background, differentiating, level columns, etc. We also need to be able to drop markers on data, look at line cuts. Lastly, appending a table of metadata (or annotated image) would be very useful for the lab notebook.

There is some work in https://github.com/qdev-dk/Qcodes/pull/57

Start by reviewing the work referenced in Qcodes pull request #57 and compare it with the requested plotting utilities. The issue mentions background subtraction, differentiation, leveling columns, markers, line cuts, and metadata or annotated-image tables; completion would require a decided scope and corresponding support for the selected items.

Written by the indexing model from the issue text.