ggplot scale for time of date only, when using POSIXct datetimes

Question

In ggplot2, I have a question about appropriate scales for making POSIXct datetimes into time-of-day in an axis. Consider:

library(tidyverse)
library(lubridate)
library(hms)
library(patchwork)

test <- tibble(
  dates = c(ymd_hms("2022-01-01 6:00:00"),
            ymd_hms("2023-01-01 19:00:00")),
  x = c(1, 2),
  hms_dates = as_hms(dates)
)

plot1 <- ggplot(test) + geom_point(aes(x = x, y = dates)) +
  scale_y_time()

plot2 <- ggplot(test) + geom_point(aes(x = x, y = hms_dates)) +
  scale_y_time()

plot1 + plot2

1. Plot 1 y axis includes dates and time, but Plot 2 shows just time of day. That's what I want! I'd like to generate plot 2 like images without having to use the hms::as_hms approach. This seems to imply some options for scale_y_datetime (or similar) that I can't discover. I'd welcome suggestions.

2. Does someone have an example of how to use the limits option in scale_*_time, or (see question #1) limits for a scale_y_datetime that specifies hours within the day, e.g. .. limits(c(8,22)) predictably fails.

Answer 1

For your second question, when dealing with dates or datetimes or times you have to set the limits and/or breaks as dates, datetimes or times too, i.e. use limits = as_hms(c("8:00:00", "22:00:00"):

library(tidyverse)
library(lubridate)
library(hms)

ggplot(test) + geom_point(aes(x = x, y = hms_dates)) +
  scale_y_time(limits = as_hms(c("8:00:00", "22:00:00")))
#> Warning: Removed 1 rows containing missing values (`geom_point()`).

Concerning your first question. TBMK this could not be achieved via scale_..._datetime. And if you just want to show the time part of your dates then converting to an has object is IMHO the easiest way to achieve that. You could of course set the units to be shown as axis text via the date_labels argument, e.g. date_labels="%H:%M:%S" to show only the time of day. However, as your dates variable is still a datetime the scale, breaks and limits will still reflect that, i.e. you only change the format of the labels and for your example data you end up with an axis showing the same time for each break, i.e. the start of the day.

ggplot(test) + geom_point(aes(x = x, y = dates)) +
  scale_y_datetime(date_labels = "%H:%M:%S")