Feature request: two alpha arguments for `stat_lineribbon`

[qdread]

Hello, I appreciate greatly all the work you've put into this package which is a cornerstone of the work I do. One thing that is a minor frustration is the way alpha is set for stat_lineribbon(). I would like to be able to independently set the transparency of the line and the ribbon. ... often it is nice to have the ribbons be more transparent than the lines, when several ribbons are overlapping. Is this an easy fix? I am not experienced with custom geoms. Thanks a lot for your work!

[clairedlb]

Hello. I am new to the package and together with @FannyMoreau and
under the supervision of [@katossky / @JMeyer31] we are willing to solve this
issue, as part of an assignment at the ENSAI school of statistical engineering
(France). We will try to be as autonomous as possible but we may ask for guidance from now and then. We
have a solid background in statistics and a reasonable one in computer science.
If any maintainer thinks this issue is too ambitious for a beginner, please tell
us so and - if you have time - guide us to a project / issue were our help may
be more useful.

[FannyMoreau]

Hello, so far we have made several attempts to resolve the issue, but none have been conclusive. We have separated the alpha parameter into two parameters, alpha_curve and alpha_ribbon, in the code of the geom_lineribbon function. However, several functions (draw_key_rect and draw_key_path) from the ggplot2 package are used in the code and take as input a object data that contains the alpha parameter. We tried assigning data the value of alpha_curve (or alpha_ribbon, depending on the case), but it still didn't work.

One idea was to code similar functions to those used from ggplot2 in our code so that we no longer use them. However, these functions themselves rely on other functions (for example, rectGrob for draw_key_rect), whose source code we couldn't find (perhaps it's not accessible).

The geom_lineribbon function is created from a GeomLineribbon class, which does not take our new alpha_curve and alpha_ribbon parameters into account. One solution was to try to include these parameters in the function and class within the function’s code without modifying the class itself. However, geom_lineribbon still does not recognize our alpha parameters. Perhaps we have coded this part incorrectly, which is why we have sought help from LLMs.

@clairedlb and Fanny Moreau

[katossky]

For your information, you can directly link to lines of code by copy-pasting the permalink from Github. It shows up like this :

ggdist/R/geom_lineribbon.R

Line 78 in ef4391a

draw_key_rect(data, params, size)

You can also quote code with "`" like this.

---

After looking into it seems like you do indeed need to change the GeomLineribbon.

Ribbons are created here :

ggdist/R/geom_lineribbon.R

Line 230 in ef4391a

ribbon_grobs = dlply_(data, grouping_columns, function(d) {

And lines just after :

ggdist/R/geom_lineribbon.R

Line 244 in ef4391a

line_grobs = dlply_(data, grouping_columns, function(d) {

Imagine that you created a supplemental alpha_ribbon parameter. I guess you could manipulate alpha_ribbon and alpha parameters from data inside GeomLineribbon. For instance, you could save both alpha parameter away from data (like alpha <- data$alpha), delete the non-standard parameter (i.e. alpha_ribbon in my example) and then recreate data$alpha <- ... with the appropriate parameter just before each call to dlply_(...). That way, you each time get only one parameter called alpha, while the content of the parameter changes.

Does it make sense ?