Dash section

[dy]

This PR resolves plotly/plotly.js#2946 and #13. The technique is straight: a segment direction is snapped to one of possible dash pattern directions, by default 8. That gives the following solutions:

3 patterns:

4 patterns:

5 patterns:

6 patterns:

7 patterns:

8 patterns (default):

9 patterns:

10 patterns:

12 patterns:

16 patterns:

20 patterns:

40 patterns:

No snapping:

That is still imperfect - pattern is broken at some joins, but that number of broken joins is reduced from every segment to sufficiently different segments.

[dy]

Possible corrections:

• detect best number of sections based on line character (tangent stats?)

• detect sections phase shift:
  
  vs
  

• exploit symmetry to align opposite direction patterns
  

• dash pattern length ←→ to min segment length ←→ thickness limitation?

[etpinard]

Great @dy Much better now: https://github.com/plotly/plotly.js/compare/dima-dash-lines

[etpinard]

cc @alexcjohnson any thoughts on Dima's fix here?

[alexcjohnson]

Very interesting strategy @dy, I like it! I think 8 patterns is a good compromise between join mismatches and dash pattern misalignment. But I'd make sure we have exact horizontal & vertical patterns, since those are very common cases and we don't want to bias viewers into thinking a horizontal line is slightly tilted

[mitja-p]

@dy is there any reason why the conflict has not been solved? This is a good idea and would fix plotly problems with dashed/dotted lines.

[dy]

@etpinard @archmoj is that still of any interest? Should me or @archmoj spend some time merging that? Not sure I'll be able to test against all plotly cases.