Feature Request - Wouldn't it be cool if ggplot2 recognized `dplyr::group_by()`?

I've got a feature request to give ggplot2 the ability to recognize dplyr::group_by().

    library(tidyverse)
    library(lubridate)
    library(stringr)
    df <- 
      tibble(Date = as.Date(0:364, origin = "2017-07-01"), Value = rnorm(365)) %>% 
      mutate(Year = str_sub(Date, 1, 4), 
             MoFloor = floor_date(Date, unit = "month")) %>% 
      group_by(Year, MoFloor) %>% 
      mutate(MoAvgValue = mean(Value)) %>% 
      ungroup() %>% 
      group_by(Year) %>% 
      mutate(MinMoFloor = min(MoFloor),
             MaxMoFloor = max(MoFloor),
             YearAvgValue = mean(MoAvgValue))

Let's first plot the data frame above.

    ggplot(df, aes(MoFloor, MoAvgValue, group = Year)) + 
      facet_grid(~Year, scale = "free_x", space = "free_x") + 
      geom_point()

In my call to the facet_grid() function I added the arguments scale = "free_x" and space = "free_x" to get rid of empty white space on the plots.

When I go ahead and add geom_segment()s based on group_by()d data, the scale = "free_x" and space = "free_x" arguments are negated. The empty white space reappears!

    ggplot(df, aes(MoFloor, MoAvgValue, group = Year)) + 
      facet_grid(~Year, scale = "free_x", space = "free_x") + 
      geom_point() + 
      geom_segment(data = df, 
                   aes(x = min(MinMoFloor), 
                       y = YearAvgValue, 
                       xend = max(MaxMoFloor), 
                       yend = YearAvgValue))

My df data frame is grouped by Year. The geom_segment() function does not recognize this when I enter (for example) the x = min(MinMoFloor) argument. geom_segment() is pulling the min(MinMoFloor) from the global column, instead of the grouped column.

I know I could substitute a modified geom_hline() function instead of geom_segment() for a working solution, but my lines are sometimes diagonal. And sometimes I'll have 50+ of these facet panels in my visualization, so defining the values manually is not practical either.

Thanks for considering this feature.

[yutannihilation]

I don't think ggplot2 can be group_by()-aware, You might feel it's convenient if it behaves differently, but someone might feel it's confusing. I'm sure it would break many existing codes (e.g. #2378)

That said, I feel there's some need for a group-aware Stat in ggplot2. I see this kind of requests and questions are posted periodically.

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