plotter: Implement XYers interface for polygons

[ctessum]

This makes it possible for external types to implement the interface without copying all data points.

Fixes #442.

Please take a look.

[codecov-io]

Codecov Report

Merging #555 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   69.28%   69.29%   +0.01%     
==========================================
  Files          52       52              
  Lines        5339     5341       +2     
==========================================
+ Hits         3699     3701       +2     
  Misses       1468     1468              
  Partials      172      172

Impacted Files	Coverage Δ
plotter/polygon.go	91.93% <100%> (ø)	:arrow_up:
plotter/plotter.go	72.22% <100%> (+0.79%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e2840ee...ccfad0c. Read the comment docs.

[codecov-io]

Codecov Report

Merging #555 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   69.28%   69.29%   +0.01%     
==========================================
  Files          52       52              
  Lines        5339     5341       +2     
==========================================
+ Hits         3699     3701       +2     
  Misses       1468     1468              
  Partials      172      172

Impacted Files	Coverage Δ
plotter/polygon.go	91.93% <100%> (ø)	:arrow_up:
plotter/plotter.go	72.22% <100%> (+0.79%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e2840ee...ccfad0c. Read the comment docs.

[codecov-io]

Codecov Report

Merging #555 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   69.28%   69.29%   +0.01%     
==========================================
  Files          52       52              
  Lines        5339     5341       +2     
==========================================
+ Hits         3699     3701       +2     
  Misses       1468     1468              
  Partials      172      172

Impacted Files	Coverage Δ
plotter/polygon.go	91.93% <100%> (ø)	:arrow_up:
plotter/plotter.go	72.22% <100%> (+0.79%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e2840ee...ccfad0c. Read the comment docs.