title: Labeling extrema with stat_peaks/stat_valleys produces duplicate labels
body: <p>I extracted some longitudinal temperature data from a .nc weather dataset (<em>ncdf4</em> package) and would like to label the local extrema with their respective dates from x-axis using <em>ggplot2</em> and its extension <em>ggpmisc</em> that includes <code>stat_peaks</code>/<code>stat_valleys</code>. Oddly, all the labels read the same: "Dec 1969".</p>
  
  <p>I figured the most likely culprit was that my data used for the x-axis was not formatted correctly as <code>Date</code>, but the x-axis displays correctly and I have checked the class of the input data to confirm. I also tried applying <code>group=1</code> which resulted in no change -- I admit I am new to R and ggplot2 (more familiar with Python/Pandas) and do not completely understand what group=1 does, though it was necessary to get the line to display correctly. Perhaps this is the result of a bug?</p>
  
  <pre><code>ggplot(df_denver, aes(x=Date, y=Temp..C., group=1)) + 
    geom_line() +
    scale_x_date(date_labels="%b %Y", date_breaks = "10 years", expand=c(0,0)) +
    stat_peaks(span=24, ignore_threshold = 0.80, color="red") +
    stat_peaks(geom="text", span=24, ignore_threshold = 0.80, x.label.fmt = "%b %Y", color="red", angle=90, hjust=-0.1) +
    stat_valleys(span=24, ignore_threshold = 0.55, color="blue") +
    stat_valleys(geom="text", span=24, ignore_threshold = 0.55, x.label.fmt = "%b %Y", color="blue", angle=90, hjust=1.1) +
    labs(x="Date", y="Temp (C)", title="Monthly Air Surface Temp for Denver from 1880 on")
  </code></pre>
  
  <p>Here are the first 100 rows of my dataset that produce 3 peaks and 3 valleys to illustrate:</p>
  
  <pre><code>          Date    Temp..C.
  1   1880-01-01  2.91287017
  2   1880-02-01 -2.73586297
  3   1880-03-01 -2.04185677
  4   1880-04-01  0.37948364
  5   1880-05-01  0.78548384
  6   1880-06-01  0.44176754
  7   1880-07-01 -1.06966007
  8   1880-08-01 -0.53162575
  9   1880-09-01 -0.29665694
  10  1880-10-01 -2.08401608
  11  1880-11-01 -9.46955109
  12  1880-12-01 -1.52052176
  13  1881-01-01 -2.53366208
  14  1881-02-01 -1.88263988
  15  1881-03-01 -0.06864686
  16  1881-04-01  3.32321167
  17  1881-05-01  1.75613177
  18  1881-06-01  2.82765651
  19  1881-07-01  1.76543093
  20  1881-08-01  1.39409852
  21  1881-09-01 -0.98141575
  22  1881-10-01 -0.63346595
  23  1881-11-01 -1.95676208
  24  1881-12-01  3.28983855
  25  1882-01-01 -0.64792717
  26  1882-02-01  2.15854502
  27  1882-03-01  2.91465187
  28  1882-04-01  0.56616443
  29  1882-05-01 -1.89441001
  30  1882-06-01 -0.63149375
  31  1882-07-01 -0.64883423
  32  1882-08-01  0.82802373
  33  1882-09-01  0.66150969
  34  1882-10-01 -0.54113626
  35  1882-11-01 -1.21310496
  36  1882-12-01  1.30559540
  37  1883-01-01 -1.41802752
  38  1883-02-01 -6.39232874
  39  1883-03-01  2.96320987
  40  1883-04-01 -0.48122203
  41  1883-05-01 -0.99614143
  42  1883-06-01 -0.67229420
  43  1883-07-01 -0.56595141
  44  1883-08-01  0.52161294
  45  1883-09-01  0.09190032
  46  1883-10-01 -2.65115738
  47  1883-11-01  1.88332438
  48  1883-12-01 -0.19942272
  49  1884-01-01 -0.34669495
  50  1884-02-01 -2.21085262
  51  1884-03-01  0.55254096
  52  1884-04-01 -1.21859336
  53  1884-05-01 -0.40969065
  54  1884-06-01  0.44454563
  55  1884-07-01  1.28881764
  56  1884-08-01 -1.09331822
  57  1884-09-01  1.52377772
  58  1884-10-01  1.76569140
  59  1884-11-01  0.72411090
  60  1884-12-01 -4.64927006
  61  1885-01-01 -1.03242493
  62  1885-02-01 -0.79325873
  63  1885-03-01  0.65910935
  64  1885-04-01 -0.10181000
  65  1885-05-01 -1.50702798
  66  1885-06-01 -1.25801849
  67  1885-07-01 -0.88433135
  68  1885-08-01 -1.18410277
  69  1885-09-01  0.15284735
  70  1885-10-01 -0.91721576
  71  1885-11-01  1.82403481
  72  1885-12-01  1.68553519
  73  1886-01-01 -4.21202993
  74  1886-02-01  2.43953681
  75  1886-03-01 -2.24947429
  76  1886-04-01 -1.22557247
  77  1886-05-01  2.66594267
  78  1886-06-01 -0.21662886
  79  1886-07-01  1.09909940
  80  1886-08-01  0.63720244
  81  1886-09-01 -0.11845125
  82  1886-10-01  0.49225059
  83  1886-11-01 -3.16969180
  84  1886-12-01  2.18220520
  85  1887-01-01  0.51427501
  86  1887-02-01 -0.69656581
  87  1887-03-01  3.96693182
  88  1887-04-01  0.92614591
  89  1887-05-01  1.66550291
  90  1887-06-01  1.88668025
  91  1887-07-01 -1.48990893
  92  1887-08-01 -0.98355341
  93  1887-09-01  0.93172997
  94  1887-10-01 -1.12551820
  95  1887-11-01  1.07798636
  96  1887-12-01 -2.15758419
  97  1888-01-01 -1.69266903
  98  1888-02-01  2.55955243
  99  1888-03-01 -1.83599913
  100 1888-04-01  3.63450384
  </code></pre>
  
  <p>As you can see, the labels produced by <code>stat_peaks</code> and <code>stat_valleys</code> are identical and not even within the range of the abbreviated data, rather than the correct dates corresponding to the x-axis.</p>
  
  <p><a href="https://i.stack.imgur.com/5wWTS.png" rel="nofollow noreferrer">Monthly Air Surface Temp for Denver from 1880 on</a></p>