R: How to plot x-axis by hour

Question

I have a dataframe df

     time value
1   08:04     0
2   08:12     0
3   08:20    60
4   08:28     0
5   08:36     0
6   08:44     0
7   08:52     0
8   09:00     0
9   09:08     0
10  09:16     0
11  09:24     0
12  09:32     0
13  09:40     0
14  09:48     0
15  09:56     0
16  10:04   100
17  10:12    49
18  10:20    49
19  10:28    49
20  10:36     0
21  10:44     0
22  10:52     0
23  11:00     0
24  11:08     0
25  11:16     0
26  11:24     0
27  11:32     0
28  11:40     0
29  11:48     0
30  11:56     0
31  12:04     0
32  12:12     0
33  12:20     0
34  12:28     0
35  12:36     0
36  12:44     0
37  12:52     0
38  13:00     0
39  13:08     0
40  13:16     0
41  13:24     0
42  13:32     0
43  13:40     0
44  13:48     0
45  13:56     0
46  14:04     0
47  14:12     0
48  14:20     0
49  14:28     0
50  14:36    91
51  14:44    44
52  14:52    43
53  15:00    43
54  15:08     0
55  15:16     0
56  15:24     0
57  15:32     0
58  15:40     0
59  15:48    42
60  15:56    41
61  16:04    41
62  16:12    41
63  16:20    41
64  16:28    42
65  16:36    42
66  16:44    42
67  16:52    42
68  17:00    42
69  17:08    42
70  17:16    42
71  17:24    42
72  17:32    41
73  17:40    41
74  17:48    41
75  17:56    41
76  18:04    40
77  18:12    40
78  18:20    40
79  18:28    39
80  18:36    39
81  18:44    37
82  18:52    37
83  19:00    37
84  19:08    37
85  19:16    37
86  19:24    33
87  19:32    33
88  19:40    33
89  19:48    34
90  19:56    34
91  20:04    34
92  20:12    50
93  20:20    67
94  20:28    36
95  20:36    36
96  20:44    36
97  20:52    36
98  21:00    36
99  21:08    37
100 21:16    39
101 21:24    41
102 21:32    42
103 21:40    44
104 21:48    46
105 21:56    47

In the above, df$time is a string and df$value is an integer.

I'd like to plot it with time as the x-axis, and for each hour to be marked. How do I do that?

So far I've guessed seq(df$time[1], df$time[length(df$time)], by = "hour") but this returns an error.

Answer 1

Base R does not support time variables for good reasons, eg, pesky quirks like DST. You need to create a datetime variable.

df$datetime <- as.POSIXct(paste0("2016-09-23 ", df$time), tz = "GMT")

library(ggplot2)
ggplot(df, aes(x = datetime, y = value)) +
  geom_point() +
  scale_x_datetime(date_labels = "%H:%M", date_breaks = "1 hour")

Answer 2

My Base R and lubridate solution:

library(lubridate)

h <- hour(hm(df$time))

plot(h, df$value)

lubridates function hm() converts the strings in df$time in hours and minutes and function h() extracts the hours.