title: R: How to plot x-axis by hour
body: <p>I have a dataframe <code>df</code></p>
  
  <pre><code>     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
  </code></pre>
  
  <p>In the above, <code>df$time</code> is a string and <code>df$value</code> is an integer.</p>
  
  <p>I'd like to plot it with time as the x-axis, and for each hour to be marked. How do I do that?</p>
  
  <p>So far I've guessed <code>seq(df$time[1], df$time[length(df$time)], by = "hour")</code> but this returns an error.</p>