`facet_wrap()` with `scales = "free_x"` squishes middle facets

まず、提供されている R/Shiny MWE を ggplot2、plotly、facet_wrap(scales = "free_x") とともに実行し、その後、Shiny の外部でリサイズの一連の操作を再現します。生成されたプロットのレイアウトと、リサイズ中のレスポンシブな挙動を調査します。中央の facet 行が期待される高さを維持し、リサイズ後に余白が現れなければ完了です。

Summary

When using facet_wrap() with scales = "free_x", the middle facets rows appear squished.

MWE

This bug is most apparent when working in Shiny contexts:

library(ggplot2)
library(shiny)
library(plotly)

ui <- fluidPage(
  plotlyOutput("p", height = "800px")
)

server <- function(input, output, session) {
  output$p <- renderPlotly({
    plt <- palmerpenguins::penguins |>
      ggplot(
        aes(x = sex)
      ) +
      geom_bar() +
      facet_wrap(
        ggplot2::vars(species, island, year),
        ncol = 2,
        axes = "all",
        scales = "free_x" # or "free"
      )

    plotly::ggplotly(plt)
  })
}

shinyApp(ui, server)

The result is an output where the first and last row of facets look correct, but all the middle facet rows are squished and there's a bunch of whitespace inbetween:

Interestingly, if you use scales = "free_y" or scales = "fixed", the bug is not triggered:

Potential Cause

I believe this is an issue with the responsivity / css of the resulting plot. You can get this to trigger outside of shiny, by doing the following:

1. Size your plot output window to result in a squished output, and run the following:

plt <- palmerpenguins::penguins |>
  ggplot(
    aes(x = sex)
  ) +
  geom_bar() +
  facet_wrap(
    ggplot2::vars(species, island, year),
    ncol = 2,
    axes = "all",
    scales = "free_x"
  )

plotly::ggplotly(plt)

Result:

2. Resize the plot window by increasing the height, and you get the weird result:

3. Key observation: if you now re-run the code from step 1, with the window height already tall, it renders correctly:

This is why I think there's responsivity / css thing going on here -- I imagine Shiny is rendering to a smaller height, then resizing to fill the space of the div. Responsivity is broken, triggering the weird result.

Thanks for looking in to this!