Start with legends, and less references

[Ploppz]

I am planning to work on the ideas in #34 and I had some changes in plotlib so I thought it could be a good idea to do a PR with those changes now, so that the next PR which builds on these changes is easier to read.

First and foremost I started implementing legends.
Notice that this is only implemented for Line so far. But in the following PR it will be implemented for Function and Scatter as well.

Secondly, I removed some & and replaced them with Box - better interface.

And also give the programmer possibility to specify max number of x and y ticks.

[milliams]

Hi @Ploppz,

This look good to me. I like the reduction in references and lifetimes which help simplify the API.

For the legends, I don't think we really want each representation to be specifying the SVG output of their legend, but rather should provide the abstract information needed (marker colour, shape etc. and label text) to to the View so that it can be in control of creating all the legend segments. In general, I'd like to keep Representations as abstract as possible with respect to rendering and treat them more as a holder for the processed information from the data source.

I'm happy to merge this since it's still all moving in the right direction for us.

Thanks a lot.