vg/vgimg: fails to draw horizontal lines with many line segments

[kortschak]

What are you trying to do?

Plot a long data series.

What did you do?

This won't run in playground due to timeout, but change the playground constant to false and it will run locally.

https://play.golang.org/p/gSINhzskCHi

What did you expect to happen?

Plot something equivalent to this (svg output).

What actually happened?

The plot is correct with svg, eps and pdf formats, but missing the horizontal segment with png, tiff and jpg, so it appears to be due to vg/vgimg. The *Canvas.Stroke method is a thin wrapper around *"github.com/fogleman/gg".Context.Stroke, so the error may there. Note also that the effect is not noticeably visible for len(data) values lower than ~4000 in the bug.go reproducer.

What version of Go and Gonum/plot are you using?

v0.9.0 caab778

Does this issue reproduce with the current master?

Yes.

[sbinet]

probably a duplicate of:

• plot: saved file missing data points #645
• Strange NewLine behaviour #559

[kortschak]

Possibly #645, but I don't think it looks like #559.

[FP-Guitar]

Ran into a similar issue than this. Followed the "Duplicate" issue and can reproduce the behaviour:
"Works with pdf and svg backend, fails with tiff".
For mit this is a real problem, because I really need to do plots with large datasets and high resolution and pdf or svg is very slow with most viewers.

[sbinet]

then this is an issue with the underlying gg package we use for the vgimg backend:

• invalid plot when increasing number of lines fogleman/gg#85

I must admit I haven't followed up and didn't dive in freetype/raster...
did you try using vg/vggio in headless mode?
as hinted there vg/vggio may give slightly better results.

[sbinet]

well... as mentioned in fogleman/gg#85 (comment) it's been a while.
I'll probably bite the bullet and fork (where? sbinet-gonum/gg? gonum/plot-vgimg-gg?) gg to fix this issue.
any suggestions/preferences as to where this should live?

[kortschak]

I don't think I can answer that because the choice is essentially an indication of where the burden will lie and that's obviously a thing for you to decide. I can say that I will try to help maintain it no matter where it lives.