在ggplot2中控制x轴上的时间刻度和binwidth

Question

Here is a dput sample of the data I am working with:

structure(list(time = structure(c(1426552275, 1426552184, 1426552085, 
1426551044, 1426550965, 1426550791, 1426550346, 1426549180, 1426549031, 
1426548975), class = c("POSIXct", "POSIXt"), tzone = "EST"), 
    location = c("South Africa,New York City", "Utah", "United States Of Africa", 
    "New York", "ATLANTA", "Atlanta, GA", "New York City!", "NYC via Chicago", 
    "Las Vegas, Nevada, USA", "Memphis TN"), uniqueid = c(5.77553e+17, 
    5.77552e+17, 5.77552e+17, 5.77548e+17, 5.77547e+17, 5.77547e+17, 
    5.77545e+17, 5.7754e+17, 5.77539e+17, 5.77539e+17)), .Names = c("time", 
"location", "uniqueid"), row.names = c(1L, 2L, 22L, 23L, 24L, 
27L, 28L, 29L, 30L, 31L), class = "data.frame")

When I plot this data with time on x-axis I get:

ggplot(data = temp, aes(x = time)) + 
  geom_bar() + 
  scale_x_datetime("time") + 
  scale_y_continuous("frequency")

When I increase the data to a 100 rows from my original dataset, I get:

Because I am not specifying binwidth, I get the message :

stat_bin: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.
Warning message:
position_stack requires constant width: output may be incorrect 

I am not able to understand this. In the charts, how much time does each bar represent? By looking at the x-axis, I can make some guess - like the first graph each bar is 1.4mins and in the second graph probably 40 mins. But how should I calculate the value to specify in the binwidth argument?

Answer 1

Copying @Gregor's comment into an answer:

Posix times ( POSIXct ) are stored in seconds, so specify a binwidth for a time duration that you want, eg, binwidth = 5 * 60