feature/requirements discussion regarding arbitrary plotting

##### Describe the issue
New feature requirements discussion (moved from [twitter](https://twitter.com/kyoneoblood/status/962501082684551168)).

> is there a way/tutorial to plot other things (like the 'expresion' tab)? I work with geometry and it'd be neat to _view_ the geometrical representation I'm debugging
.
I've worked on a gdb.Command that parses polygon points from its gdb.Value repr (let's say it's originally a boost::polygon_data). We take those points, feed them to plotly and write the plot to html file since we have the data as html (probably as an iframe) we might as well render it live in a gdbgui tab (as the 'expression' tab)

There is no way to plot anything else in the expressions tab, so no tutorial exists. But the feature request is not totally clear to me. Could this be distilled further into a requirement for gdbgui to perform? For example,

* gdbgui should have an input next to each expression to enter an arbitrary URL to display an iframe inline with an expression

or

* gdbgui should allow for optional streaming data value logging to a file of type (JSON? pickle? some other binary format?)

or

* gdbgui should provide an input to compute and plot values of gdb variables, for example `x * y`, where x and y are both gdb variables that could be plotted on their own

or

* gdbgui should provide a way to define the value used to plot on the x axis, rather than always use data point count on the x axis

or

* Something else. These examples are not meant to be limiting, just to throw some examples out there. I am interested in what features others might find useful, and more on what you/others have in mind.

Start with the expressions tab and the alternative requirements listed in the issue, then review the existing plotting behavior described there. Before implementation, turn the discussion into one agreed requirement with a defined input, output, and completion criterion.

Written by the indexing model from the issue text.