Feature request: expose `shape` argument in `geom_curve()`

[fmarotta]

Hi! Currently geom_segment() supports one variant, geom_curve(), to connect two points with a curved line. I would find it useful to have in base ggplot an "elbow" geom, that connects points with a, well, elbow-shaped line, similar to geom_step() but with the same aesthetics as geom_segment() and geom_curve(). I am aware of geom_edge_elbow() in ggraph, but I believe this geom can be used more generally.

[teunbrand]

Hi there, thanks for your suggestion!
Generally we recommend using the extensions system for innovation of new stats and geoms and keep ggplot2 contained to core functionality.
The elbow geom does not seem unreasonably hard to implement, nor does it appear to require any change in ggplot2 to be implemented, so this seems to fit the purpose of the extension system like a glove.
For this reason, I suggest proposing this elsewhere in an extension package (or start one of your own).

[fmarotta]

Ahh, I see, thanks for your reply. It seems a pity to scatter such simple and basic functionality to yet another package, I'll try to move this issue to ggforce, which already has some nice geoms for connecting points. I'm also happy to prepare a MR.

[ssp3nc3r]

If I understand your description, It seems that ggalt::geom_xspline may already provide something similar.

[fmarotta]

I was more looking for something with x, y, xend, yend aesthetics, but I'll look into geom_xspline, thanks for the suggestion!

[teunbrand]

I realised that this doesn't necessarily needs to be a new geom, it essentially can be done by exposing the shape argument via geom_curve().

library(grid)
grob <- curveGrob(x1 = 0.2, x2 = 0.8, y1 = 0.2, y2 = 0.8, shape = 0, curvature = -1)
grid.newpage()
grid.draw(grob)

^{Created on 2025-06-10 with reprex v2.1.1}

[Breeze-Hu]

If geom_curve is made optimised, is there any chance of implementing something like geom_xspline natively in ggplot2: https://yonicd.github.io/ggalt/reference/geom_xspline.html

It may be possible to plot curves and points more elegantly (with some loss of precision, of course) in dealing with some time series. Instead it's all spikes.

[teunbrand]

Changing the implementation of geom_curve() altogether is out of scope for this issue. Also: no, if it is implemented by an extension there is generally speaking no need to implement it in ggplot2: that is the whole point of the extension system.

[Breeze-Hu]

Okay. Thanks for your reply : ）
I am aware of this principle you mentioned (ggplot2 is already mature and more needs try to be realised through extensions rather than adding new features).

There may be features in extensions that are scattered across many packages, some of which haven't been updated in a long time, and the maintainers may be close to giving up on maintaining them, and will be ‘lost’ once ggplot2 undergoes a major revision. That's what I'm worried about ...... In a sense, this also affects the ecology of ggplot2.

[teunbrand]

In such cases we can only hope that the abandoned package is MIT licenced and a motived person can take on maintainership of certain functions themselves. Another option is to community-organise a (collection of) package(s) where maintainers can turn over as their priorities evolve.

[fmarotta]

Thanks for reopening the issue! Exposing shape would be great and it would make geom_curve() much more flexible and general. Since we're already discussing changes, is there any chance to promote both shape and curvature to aesthetics? Currently, curvature must be of length 1, which forces me to split the data into multiple geoms when I want to draw lines with different curvature.