title: Axis labels on two lines with nested x variables (year below months)
body: <p>I would like to display months (in abbreviated form) along the horizontal axis, with the corresponding year printed once. I know how to display month-year: </p>
  
  <p><a href="https://i.stack.imgur.com/3Koi6.png" rel="noreferrer"><img src="https://i.stack.imgur.com/3Koi6.png" alt="enter image description here"></a></p>
  
  <p>The un-needed repetition of the year clutters the labels. Instead I would like something like this:</p>
  
  <p><a href="https://i.stack.imgur.com/W9DUw.png" rel="noreferrer"><img src="https://i.stack.imgur.com/W9DUw.png" alt="enter image description here"></a></p>
  
  <p>except that the year would be printed <strong>below</strong>  the months.</p>
  
  <p>I printed the year above the axis labels, because that's the best I could do. This follows a limitation of the <code>annotate()</code> function, which gets clipped if it lies outside of the plot area. I am aware of possible workarounds based on <code>annotate_custom()</code>, but I couldn't make them to work with date objects (I did not try to convert dates to numbers and back to dates again, as it seemed more complicated than hopefully necessary)</p>
  
  <p>I'm wondering if the new <code>dup_axis()</code> could be hijacked for this purpose. If instead of sending the duplicated axis to the opposite side of the panel, it could send it a few lines below the duplicated axis, then perhaps it would just be a matter of setting up one axis with <code>panel.grid.major</code> blanked out and the labels set to <code>%b</code>,  while the other axis would have <code>panel.grid.minor</code> blanked out and the labels set to <code>%Y</code>. (an added challenge is that the year labels would be shifted to October instead of January)</p>
  
  <p>These questions are related. However, the <code>annotate_custom()</code> function and <code>textGrob()</code> functions do not play well with dates, as far as I can tell.</p>
  
  <p><a href="https://stackoverflow.com/questions/31079210/how-can-i-add-annotations-below-the-x-axis-in-ggplot2">how-can-i-add-annotations-below-the-x-axis-in-ggplot2</a></p>
  
  <p><a href="https://stackoverflow.com/questions/10014187/displaying-text-below-the-plot-generated-by-ggplot2">displaying-text-below-the-plot-generated-by-ggplot2</a></p>
  
  <p>Data and basic code below:</p>
  
  <pre><code>    library("ggplot2")
      library("scales")
      ggplot(data = df, aes(x = Date, y = value)) + geom_line() +
          scale_x_date(date_breaks = "2 month", date_minor_breaks = "1 month", labels = date_format("%b %Y")) +
          xlab(NULL)
  
      ggplot(data = df, aes(x = Date, y = value)) + geom_line() +
          scale_x_date(date_minor_breaks = "2 month", labels = date_format("%b")) +   
          annotate(geom = "text", x = as.Date("1719-10-01"), y = 0, label = "1719") +
          annotate(geom = "text", x = as.Date("1720-10-01"), y = 0, label = "1720") +
          xlab(NULL)
  
  
      # data
      df &lt;- structure(list(Date = structure(c(-91455, -91454, -91453, -91452, 
      -91451, -91450, -91448, -91447, -91446, -91445, -91444, -91443, 
      -91441, -91440, -91439, -91438, -91437, -91436, -91434, -91433, 
      -91431, -91430, -91429, -91427, -91426, -91425, -91424, -91423, 
      -91422, -91420, -91419, -91418, -91417, -91416, -91415, -91413, 
      -91412, -91411, -91410, -91409, -91408, -91406, -91405, -91404, 
      -91403, -91402, -91401, -91399, -91398, -91397, -91396, -91395, 
      -91394, -91392, -91391, -91390, -91389, -91388, -91387, -91385, 
      -91384, -91382, -91381, -91380, -91379, -91377, -91376, -91375, 
      -91374, -91373, -91372, -91371, -91370, -91369, -91368, -91367, 
      -91366, -91364, -91363, -91362, -91361, -91360, -91359, -91357, 
      -91356, -91355, -91354, -91353, -91352, -91350, -91349, -91348, 
      -91347, -91346, -91345, -91343, -91342, -91341, -91340, -91339, 
      -91338, -91336, -91335, -91334, -91333, -91332, -91331, -91329, 
      -91328, -91327, -91326, -91325, -91324, -91322, -91321, -91320, 
      -91319, -91315, -91314, -91313, -91312, -91311, -91310, -91308, 
      -91307, -91306, -91305, -91304, -91303, -91301, -91300, -91299, 
      -91298, -91297, -91296, -91294, -91293, -91292, -91291, -91290, 
      -91289, -91287, -91286, -91285, -91284, -91283, -91282, -91280, 
      -91279, -91278, -91277, -91276, -91275, -91273, -91272, -91271, 
      -91270, -91269, -91268, -91266, -91265, -91264, -91263, -91262, 
      -91261, -91259, -91258, -91257, -91256, -91255, -91254, -91252, 
      -91251, -91250, -91249, -91248, -91247, -91245, -91244, -91243, 
      -91242, -91241, -91240, -91238, -91237, -91236, -91235, -91234, 
      -91233, -91231, -91230, -91229, -91228, -91227, -91226, -91224, 
      -91223, -91222, -91221, -91220, -91219, -91217, -91216, -91215, 
      -91214, -91213, -91212, -91210, -91209, -91208, -91207, -91205, 
      -91201, -91200, -91199, -91198, -91196, -91195, -91194, -91193, 
      -91192, -91191, -91189, -91188, -91187, -91186, -91185, -91184, 
      -91182, -91181, -91180, -91179, -91178, -91177, -91175, -91174, 
      -91173, -91172, -91171, -91170, -91168, -91167, -91166, -91165, 
      -91164, -91163, -91161, -91160, -91159, -91158, -91157, -91156, 
      -91154, -91153, -91152, -91151, -91150, -91149, -91147, -91146, 
      -91145, -91144, -91143, -91142, -91140, -91139, -91138, -91131, 
      -91130, -91129, -91128, -91126, -91125, -91124, -91123, -91122, 
      -91121, -91119, -91118, -91117, -91116, -91115, -91114, -91112, 
      -91111, -91110, -91109, -91108, -91107, -91104, -91103, -91102, 
      -91101, -91100, -91099, -91097, -91096, -91095, -91094, -91093, 
      -91091, -91090, -91089, -91088, -91087, -91086, -91084, -91083, 
      -91082, -91081, -91080, -91079, -91077, -91076, -91075, -91074, 
      -91073, -91072, -91070, -91069, -91068, -91065, -91063, -91062, 
      -91061, -91060, -91059, -91058, -91056, -91055, -91054, -91053, 
      -91052, -91051, -91049, -91048, -91047, -91046, -91045, -91044, 
      -91042, -91041, -91040, -91039, -91038, -91037, -91035, -91034, 
      -91033, -91032, -91031, -91030, -91028, -91027, -91026, -91025, 
      -91024, -91023, -91021, -91020, -91019, -91018, -91017, -91016, 
      -91014, -91013, -91012, -91011, -91010, -91009, -91007, -91006, 
      -91005, -91004, -91003, -91002, -91000, -90999, -90998, -90997, 
      -90996, -90995, -90993, -90992, -90991, -90990, -90989, -90988, 
      -90986, -90985, -90984, -90983, -90982), class = "Date"), value = c(113, 
      113, 113, 113, 114, 114, 114, 115, 115, 115, 116, 116, 116, 116, 
      117, 117, 117, 117, 116, 117, 116, 116, 116, 117, 117, 117, 117, 
      117, 117, 117, 116, 117, 116, 116, 116, 117, 117, 117, 117, 117, 
      117, 117, 116, 116, 117, 117, 117, 117, 117, 117, 117, 117, 117, 
      117, 117, 118, 118, 118, 118, 117, 118, 117, 117, 117, 117, 117, 
      117, 118, 116, 116, 116, 116, 116, 116, 116, 117, 117, 118, 118, 
      118, 118, 118, 119, 120, 120, 119, 119, 120, 120, 121, 121, 122, 
      124, 124, 122, 123, 124, 123, 123, 123, 123, 123, 124, 124, 126, 
      126, 126, 126, 126, 125, 125, 126, 127, 126, 126, 125, 126, 126, 
      126, 128, 128, 128, 130, 133, 131, 133, 134, 134, 134, 136, 136, 
      136, 135, 135, 135, 136, 136, 136, 136, 135, 135, 135, 135, 130, 
      129, 129, 130, 131, 136, 138, 155, 157, 161, 170, 174, 168, 165, 
      169, 171, 181, 184, 182, 179, 181, 179, 175, 177, 177, 174, 170, 
      174, 173, 178, 173, 178, 179, 182, 184, 184, 180, 181, 182, 182, 
      184, 184, 188, 195, 198, 220, 255, 275, 350, 310, 315, 320, 320, 
      316, 300, 310, 310, 320, 317, 313, 312, 310, 297, 285, 285, 286, 
      288, 315, 328, 338, 344, 345, 352, 352, 342, 335, 343, 340, 342, 
      339, 337, 336, 336, 342, 347, 352, 352, 351, 352, 352, 351, 352, 
      352, 355, 375, 400, 452, 487, 476, 475, 473, 485, 500, 530, 595, 
      720, 720, 770, 750, 770, 750, 735, 740, 745, 735, 700, 700, 750, 
      760, 755, 755, 760, 760, 765, 950, 950, 950, 875, 875, 875, 880, 
      880, 880, 900, 900, 900, 880, 880, 890, 895, 890, 880, 870, 870, 
      870, 870, 870, 860, 860, 860, 860, 850, 840, 810, 820, 810, 810, 
      805, 810, 805, 820, 815, 820, 805, 790, 800, 780, 760, 765, 750, 
      740, 820, 810, 800, 800, 775, 750, 810, 750, 740, 700, 705, 660, 
      630, 640, 595, 590, 570, 565, 535, 440, 400, 410, 400, 405, 390, 
      370, 300, 300, 180, 200, 310, 290, 260, 260, 275, 260, 270, 265, 
      255, 250, 210, 210, 200, 195, 210, 215, 240, 240, 220, 220, 220, 
      220, 210, 212, 208, 220, 210, 212, 208, 220, 215, 220, 214, 214, 
      213, 212, 210, 210, 195, 195, 160, 160, 175, 205, 210, 208, 197, 
      181, 185)), .Names = c("Date", "value"), row.names = c(NA, 393L
      ), class = "data.frame")
  </code></pre>