format y axis for plotting time of day with qplot/ggplot2

Question

I have many data entries with two properties, the time of day (24h format, UTC, no date given) and a string containing the type of data.

I want to visualize the distribution of entries along the time axis for each type.

Consider the following example code:

library(ggplot2)

time <- c("22:12", "11:04", "00:04", "23:45", "12:04", "16:33")
type <- c("Foo", "Bar", "Foo", "Foo", "Foo", "Bar")

data <- data.frame(time, type)

qplot(data$type, strptime(data$time, "%H:%M")) +
  scale_y_datetime(date_breaks="1 hour", date_labels="%H:%M") + xlab("Type") + ylab("Time")

I want the chart to display the type on the x axis and the y axis to be in the range from 00:00 to 23:59 (no matter what actual times are in the dataset).

Not only does the time ranges from 21.00 to 22.00 (overflowing once), but the single entries are also displayed wrong (just look at the two 'Bar' entries):

Do you have any idea why the axis labels are that strange, and why the time it not plotted at the correct position? Maybe a timezone issue?

If you take away the scale_y_datetime, I think it plots correctly (but now has the date shown too) — Richard Telford
O try to change time variable format from factor to POSIXct. — AndriusZ

Richard Telford Richard Telford · Accepted Answer · 2016-04-11T12:31:01

Since scales:::date_format takes UTC by default and I cannot see how to change this argument through scale_y_datetime, the simplest solution is probably to specify that you are using UTC whatever your real timezone.

qplot(data$type, strptime(data$time, "%H:%M", tz = "UTC")) +
  scale_y_datetime(date_breaks="1 hour", date_labels="%H:%M", expand = c(0,0)) +
  xlab("Type") + ylab("Time")

format y axis for plotting time of day with qplot/ggplot2

1 Answers