Allow geoms to ignore some set aesthetics

[jiho]

Let us say I want to combine a few geoms to make a specific plot, I can define such a function:

library("ggplot2")

d <- data.frame(x=rnorm(10), y=rnorm(10), a=letters[1:10])

my_plot <- function(data, mapping, ...) {
  ggplot() +
    geom_line(data=data, mapping=mapping, ...) +
    geom_point(data=data, mapping=mapping, ...)
}

my_plot(d, aes(x=x, y=y))

But then, if I want to change the linetype, geom_point throws an error because linetype is meaningless for it:

my_plot(d, aes(x=x, y=y), linetype="dashed")

Error: Unknown parameters: linetype

If I recall correctly, this was just silently ignored in the past. Interestingly, mapped aesthetics do not throw this error:

my_plot(d, aes(x=x, y=y, linetype="dashed"))

But the result is pretty useless of course ;-). A more useful example would be

my_plot <- function(data, mapping, ...) {
  ggplot() +
    geom_line(data=data, mapping=mapping, ...) +
    geom_point(data=data, mapping=mapping, ...) +
    geom_text(data=data, mapping=mapping, ...)
}
my_plot(d, aes(x=x, y=y, label=a))

Still moving the text up is not possible after the fact, with something like:

my_plot(d, aes(x=x, y=y, label=a), vjust=0)

I understand the rationale in telling the user explicitly about useless aesthetics, but:

• I think throwing an error is a bit too much, a warning would be enough. Stopping the execution of the function makes it very cumbersome to use ggplot programmatically to build new visualisations (the "proper" way may be to build new geoms but this requires a lot of sophistication for a simple goal).
• Alternatively (or in addition) there should be a way to easily bypass the warning/errors, through an argument, FALSE by default (if a warning is used, the person creating the function can also wrap the call in suppressWarnings and that would help).
• This should be consistent between set and mapped aesthetics (but probably ignore useless inherited aesthetics without warning).

PS: The rationale for this comment is that I often combine geoms in autoplot but want to retain the flexibility of mapping/setting aesthetics from the higher level function.

[faizan-khan-iit]

I also run into the same error when I try to run this example:

library("ggplot2")

viz <- list(iris=ggplot()+
    geom_point(aes(Petal.Width, Sepal.Length, showSelected=Species),
               data=iris, chunk_vars=character()))

Error: Unknown parameters: chunk_vars

This does enforce the point that spelling mistakes and other minor errors will be easily caught and avoided. But this also breaks the packages like Animint, that rely on the use additional parameters (for e.g. chunk_vars in the above example)

I agree with @jiho in that we should be able to bypass this error by specifying a parameter so that we may only get a warning about additional params. It will ensure that the existing packages work with little modifications, all the while achieving your intended goal of not allowing user to make minor mistakes.

I hope you give this a thought.

[tdhock]

Have you decided how to address this issue? Is there a PR? Or has the feature already been implemented?

[thomasp85]

@hadley how do you want this? An ignore.extra argument set to FALSE by default which would make the layer silently ignore extra parameters when switched on?

[hadley]

I think there are two parts to the problem:

• Informing ggplot2 about new aesthetics. I think a reasonable solution here would be to provide an API for packages to register aesthetics - i.e. on load, a animInt would call ggplot2::register_aesthetic("chunk_vars")).
• Making it easier to construct multilayer plots with varying aesthetics. Here an ignore.extra parameter to the layer seems reasonable. The problem is that adding a new parameter to layer() involves also adding it to every layer function (e.g. geom_point(), geom_histogram(), ... So I'd like to think about alternative approaches

I don't like the idea of changing the error to a warning because it doesn't fix the underlying problem.

[tdhock]

or maybe ggplot2::register_param("chunk_vars")? in animint chunk_vars is a parameter, not an aesthetic. For example geom_point(aes(x=foo, y=bar, showSelected=baz), chunk_vars=character())

[tdhock]

another alternative would be to introduce a new parameter for storing any/all arbitrary layer-specific meta-data.

for example geom_point(aes(x=foo, y=bar, showSelected=baz), meta=list(chunk_vars=character()))

the desired behavior would be that ggplot2 will recognize the meta parameter, store that list, and NOT stop with an error.

[thomasp85]

I think the discussion has now diverged som much from the original (ignoring unknown aesthetics) that it warrants its own issue...

[hadley]

Yes, let's move the extra parameters to a separate issue. It's not clear to me that you should be allowed to add extra parameters to a geom that you didn't create.