plotter: implement GlyphBoxer for plotter.Line

[sbinet]

Fixes #761.

Please take a look.

[sbinet]

one existential question: should we create one gigantic glyphbox (or rather 4, one around each of the xmin, xmax, ymin, and ymax points), or one for each of line's points (as is done in this PR) ?

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.50 :tada:

Comparison is base (1cd1f0e) 73.34% compared to head (49a83ba) 74.85%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #762 +/- ## ========================================== + Coverage 73.34% 74.85% +1.50% ========================================== Files 56 58 +2 Lines 5159 7368 +2209 ========================================== + Hits 3784 5515 +1731 - Misses 1191 1657 +466 - Partials 184 196 +12 ``` | [Impacted Files](https://codecov.io/gh/gonum/plot/pull/762?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum) | Coverage Δ | | |---|---|---| | [plotter/line.go](https://codecov.io/gh/gonum/plot/pull/762?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-cGxvdHRlci9saW5lLmdv) | `92.12% <100.00%> (+1.32%)` | :arrow_up: | ... and [54 files with indirect coverage changes](https://codecov.io/gh/gonum/plot/pull/762/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum)

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[kortschak]

I think what you have is probably the best approach. I am concerned about the impact on the step plot though. It does weird things to the alignment in ways that I would not expect.