Shift x-axis ticks with scale_x_datetime and geom_col so bars are NOT centered on tick mark

Question

I am plotting time series data in half-hour intervals, using geom_col() to show number of birds counted in each interval. ggplot2 plots each bar centered over the x-axis tick mark, but I need each bar to begin to the right of each tick mark. In other words, I need each bar to span the width of its corresponding half-hour interval.

I have tried the suggested solutions in these posts without luck:

• Bars starting after x-axis tickmark when using scale_x_datetime
• How to force the x-axis tick marks to appear at the end of bar in heatmap graph?

Below is example code that reproduces the issue:

    ``` r
library(lubridate)
#> 
#> Attaching package: 'lubridate'
#> The following objects are masked from 'package:base':
#> 
#>     date, intersect, setdiff, union
library(ggplot2)
library(tidyverse)

df <- data.frame(
  date = c("2019-05-16", "2019-05-16", "2019-05-16", "2019-05-16", "2019-05-16", "2019-05-16", "2019-05-16", "2019-05-16"),
  time = c("16:30:00", "17:00:00", "17:30:00", "18:00:00", "18:30:00", "19:00:00", "19:30:00", "20:00:00"),
  count = c(5, 100, 14, 342, 59, 321, 44, 98),
  stringsAsFactors = FALSE)

datetime_df <- df %>% 
  mutate(time_stamp = paste(date, time) %>% as_datetime())

plot <- ggplot(datetime_df, aes(x = time_stamp, y = count)) +
  geom_col() +
  scale_x_datetime(breaks = scales::date_breaks("30 mins"), date_labels = "%H:%M",
                                    limits = c(as_datetime("2019-05-16 16:00:00"), 
                                               as_datetime("2019-05-16 20:30:00"))) +
  scale_y_continuous(expand = c(0,0), breaks = seq(0, 500, by = 50), limits = c(0,500))

^{Created on 2020-10-01 by the reprex package (v0.3.0)}

Here is the resulting bar chart

Many thanks for any ideas on how to solve this issue!

Module

By the way, welcome to the site! Great job having your first question be clear, obviously researched, and include a reproducible example! – Charisecharisma

Thank you! You made my first experience on the site very rewarding -- drives home the power of providing reproducible examples! Thank you again for your time. – Module

Answer 1

You can mimic this easily by adding 15 minutes to your times:

plot <- ggplot(datetime_df, aes(x = time_stamp + minutes(15), y = count)) +
  geom_col() +
  scale_x_datetime(breaks = scales::date_breaks("30 mins"), date_labels = "%H:%M",
                                    limits = c(as_datetime("2019-05-16 16:00:00"), 
                                               as_datetime("2019-05-16 20:30:00"))) +
  scale_y_continuous(expand = c(0,0), breaks = seq(0, 500, by = 50), limits = c(0,500))
plot

To make the ticks line up exactly with the start of the bar, you'll need to set the width parameter in geom_col to match the number of minutes added. After a little trial and error, this seems to work:

half_width_in_min = 13
plot <- ggplot(datetime_df, aes(x = time_stamp + minutes(half_width_in_min ), y = count)) +
  geom_col(width = 60 * 24 * 1.25 * half_width_in_min / 15) +
  scale_x_datetime(breaks = scales::date_breaks("30 mins"), date_labels = "%H:%M",
                                    limits = c(as_datetime("2019-05-16 16:00:00"), 
                                               as_datetime("2019-05-16 20:30:00"))) +
  scale_y_continuous(expand = c(0,0), breaks = seq(0, 500, by = 50), limits = c(0,500))
plot

Charisecharisma

Thank you so much, @GregorThomas !! Your first suggestion to add 15 minutes to the times worked beautifully. I really appreciate your help! – Module