Style empty points in line charts via CSS

[panthony]

An up to date version of #1992

I'll merge this in my fork because it looks neat, I'll leave you to decide if you want this in the main repository.

The culprit is that currentColor is not supported by IE8, in my case I do not care, maybe some will.

Although I'm not sure how well IE8 is supported at the moment (maybe not at all.).

[codecov-io]

Codecov Report

Merging #2693 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2693   +/-   ##
=======================================
  Coverage   81.02%   81.02%           
=======================================
  Files          59       59           
  Lines        4633     4633           
=======================================
  Hits         3754     3754           
  Misses        879      879

Impacted Files	Coverage Δ
src/shape.line.js	71.75% <ø> (ø)	⬆️

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[codecov-io]

Codecov Report

Merging #2693 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2693   +/-   ##
=======================================
  Coverage   81.02%   81.02%           
=======================================
  Files          59       59           
  Lines        4633     4633           
=======================================
  Hits         3754     3754           
  Misses        879      879

Impacted Files	Coverage Δ
src/shape.line.js	71.75% <ø> (ø)	⬆️

---

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 4163be5...3c07c17. Read the comment docs.

[kt3k]

Thanks. This makes sense to me.

I think we can safely ignore IE <= 10 now.

I feel some websites still try to support IE11, but in that case we should recommend the use of c3 v0.4.24 because D3 v5 already dropped the support of IE, so c3 v0.4.24 was probably the last version which supported IE well. I think it's ok for the latest version of c3 to only support modern browsers.