title: ggplot2 boxplot only shows vertical lines
body: <p>I was trying to plot some index values as a boxplot against the values of another index, but something went wrong and I can't figure out what.</p>
  
  <p>Here's how my data looks:</p>
  
  <pre><code>'data.frame':   150 obs. of  11 variables:
   $ method             : chr  "GEMAX" "GEMAX" "GEMAX" "GEMAX" ...
   $ season_year        : chr  "autumn 2016" "autumn 2016" "autumn 2016" "autumn 2016" ...
   $ station            : chr  "1" "1" "1" "1" ...
   $ sample             : chr  "1" "2" "3" "4" ...
   $ method_year        : chr  "GE_2016" "GE_2016" "GE_2016" "GE_2016" ...
   $ method_year_station: chr  "GE_2016_1" "GE_2016_1" "GE_2016_1" "GE_2016_1" ...
   $ sample_id          : chr  "GE2016_st1_1" "GE2016_st1_2" "GE2016_st1_3" "GE2016_st1_4" ...
   $ year_station       : chr  "autumn 2016 1" "autumn 2016 1" "autumn 2016 1" "autumn 2016 1" ...
   $ Index              : Factor w/ 3 levels "H'","BQI","BBI": 1 1 1 1 1 1 1 1 1 1 ...
   $ index_value        : num  0.897 0.712 0.465 0.394 0.618 ...
   $ PLI                : num  1.54 1.54 1.54 1.54 1.54 ...
  </code></pre>
  
  <p>Here's the code I used to create my boxplot:</p>
  
  <pre><code>colour_palette_stations &lt;- c("1" = "#E69F00", "2" ="#56B4E9", "3" = "#009E73", "4" = "#0072B2", "5" = "#D55E00", "6" = "#CC79A7")
  plot &lt;- ggplot() +
    geom_boxplot(data = melted_indices, aes(x = PLI, y = index_value, fill = station)) +
    scale_fill_manual(name = "Stations", values = colour_palette_stations) +
    ylab("index value") +
    facet_wrap(~Index, scales = "free")
  plot
  </code></pre>
  
  <p>And here is the end result:
  <a href="https://i.stack.imgur.com/VuQ1n.png" rel="nofollow noreferrer">vertical lines "boxplot"</a></p>
  
  <p>I compared my code to other boxplots I've made, and it looks the same to me. Can anyone spot what I did wrong?</p>
  
  <p>EDIT: Output from</p>
  
  <pre><code>dput(melted_indices[, c("PLI", "index_value", "station")])
  </code></pre>
  
  <p>as requested:</p>
  
  <pre><code>structure(list(PLI = c(1.53772710912303, 1.53772710912303, 1.53772710912303, 
  1.53772710912303, 1.53772710912303, 3.18386506979715, 3.18386506979715, 
  3.18386506979715, 3.18386506979715, 3.18386506979715, 1.07451906856368, 
  1.07451906856368, 1.07451906856368, 1.07451906856368, 1.07451906856368, 
  1.23289254801444, 1.23289254801444, 1.23289254801444, 1.23289254801444, 
  1.23289254801444, 1.17007093273721, 1.17007093273721, 1.17007093273721, 
  1.17007093273721, 1.17007093273721, 1.22989566083404, 1.22989566083404, 
  1.22989566083404, 1.22989566083404, 1.22989566083404, 1.53772710912303, 
  1.53772710912303, 1.53772710912303, 1.53772710912303, 1.53772710912303, 
  1.07451906856368, 1.07451906856368, 1.07451906856368, 1.07451906856368, 
  1.07451906856368, 1.17007093273721, 1.17007093273721, 1.17007093273721, 
  1.17007093273721, 1.17007093273721, 1.22989566083404, 1.22989566083404, 
  1.22989566083404, 1.22989566083404, 1.22989566083404, 1.53772710912303, 
  1.53772710912303, 1.53772710912303, 1.53772710912303, 1.53772710912303, 
  3.18386506979715, 3.18386506979715, 3.18386506979715, 3.18386506979715, 
  3.18386506979715, 1.07451906856368, 1.07451906856368, 1.07451906856368, 
  1.07451906856368, 1.07451906856368, 1.23289254801444, 1.23289254801444, 
  1.23289254801444, 1.23289254801444, 1.23289254801444, 1.17007093273721, 
  1.17007093273721, 1.17007093273721, 1.17007093273721, 1.17007093273721, 
  1.22989566083404, 1.22989566083404, 1.22989566083404, 1.22989566083404, 
  1.22989566083404, 1.53772710912303, 1.53772710912303, 1.53772710912303, 
  1.53772710912303, 1.53772710912303, 1.07451906856368, 1.07451906856368, 
  1.07451906856368, 1.07451906856368, 1.07451906856368, 1.17007093273721, 
  1.17007093273721, 1.17007093273721, 1.17007093273721, 1.17007093273721, 
  1.22989566083404, 1.22989566083404, 1.22989566083404, 1.22989566083404, 
  1.22989566083404, 1.53772710912303, 1.53772710912303, 1.53772710912303, 
  1.53772710912303, 1.53772710912303, 3.18386506979715, 3.18386506979715, 
  3.18386506979715, 3.18386506979715, 3.18386506979715, 1.07451906856368, 
  1.07451906856368, 1.07451906856368, 1.07451906856368, 1.07451906856368, 
  1.23289254801444, 1.23289254801444, 1.23289254801444, 1.23289254801444, 
  1.23289254801444, 1.17007093273721, 1.17007093273721, 1.17007093273721, 
  1.17007093273721, 1.17007093273721, 1.22989566083404, 1.22989566083404, 
  1.22989566083404, 1.22989566083404, 1.22989566083404, 1.53772710912303, 
  1.53772710912303, 1.53772710912303, 1.53772710912303, 1.53772710912303, 
  1.07451906856368, 1.07451906856368, 1.07451906856368, 1.07451906856368, 
  1.07451906856368, 1.17007093273721, 1.17007093273721, 1.17007093273721, 
  1.17007093273721, 1.17007093273721, 1.22989566083404, 1.22989566083404, 
  1.22989566083404, 1.22989566083404, 1.22989566083404), index_value = c(0.897090619981485, 
  0.711951622706353, 0.46468506468643, 0.394140299815468, 0.618342434570252, 
  0.732208541750694, 0.771144765343957, 0.738719220626546, 0.714795171885277, 
  0.63193436646543, 0.360062440635905, 0.639064876705551, 0.309470371928486, 
  0.716747696239186, 0.57088671777535, 0.556100591377527, 0.790546875085912, 
  0.212924224619291, 0.655715668736619, 0.122465013397318, 0.560475196336792, 
  0.76374480870866, 0.682037160721369, 0.712266111481877, 0.517847981778762, 
  0.807939742621269, 0.887918757951212, 0.591563201935165, 0.540286924182427, 
  0.613038025183085, 0.552944939512382, 0.436835521475935, 0.444961266186975, 
  0.55980402078392, 0.657561454415399, 0.757651760773378, 0.674035613938481, 
  0.585690708435859, 0.673453720038359, 0.566623837226683, 0.738754225841618, 
  0.781074907362021, 0.717555674113688, 0.676230731907715, 0.544005131138556, 
  0.477135189884872, 0.568129212774203, 0.634005198483357, 0.501294889793803, 
  0.682679620240528, 7.73028215031384, 6.50736954661017, 4.74815959988244, 
  4.76955926408783, 5.78781958599735, 4.34823995020612, 4.36087197047454, 
  4.4585785017997, 3.96532366566562, 4.48632544264735, 4.3348816375348, 
  6.30916040409802, 3.8126333411174, 6.14455304188506, 5.78697653434632, 
  4.67125473741412, 7.19786102820837, 2.95802508008563, 4.73267592832875, 
  2.49180383843773, 5.00230342117648, 6.93222660267001, 8.71659382276766, 
  6.83380955704985, 4.75420654883516, 8.71942957192637, 9.34483234519417, 
  5.92889854043036, 4.62275111044526, 5.442201480734, 4.47217508348135, 
  2.82593192834848, 3.40169709876277, 3.90385978126599, 5.21220443365182, 
  6.80273064909952, 6.01946935605469, 5.54545399505807, 6.6354167765275, 
  4.60553994532868, 7.71006895018938, 8.71942957192637, 6.86484240640667, 
  6.05722967766911, 4.5752542267714, 3.64902653901661, 4.5243007445229, 
  4.51793843939088, 4.30813146662702, 4.57644818126768, 0.458728491050711, 
  0.36401613713986, 0.262134620587481, 0.24041212559615, 0.312077588938048, 
  0.288280165129868, 0.298690312388965, 0.300367625322683, 0.270264693071377, 
  0.281564392922554, 0.218474429961861, 0.359598593095251, 0.182087114447219, 
  0.35701701152291, 0.320122396809523, 0.273271031084653, 0.415755527830253, 
  0.137359209124409, 0.285626236719737, 0.0967782960031763, 0.273538929715971, 
  0.395717087076169, 0.445772111088459, 0.376803376197474, 0.258164791615629, 
  0.480326447116088, 0.527197439725643, 0.322986879409089, 0.263688294229414, 
  0.307314073849529, 0.252672106134595, 0.164608843281875, 0.186595762734099, 
  0.238811635258751, 0.310869644258428, 0.389815919043416, 0.339249683737036, 
  0.298432829669081, 0.363215178362548, 0.259909371410129, 0.422965903343036, 
  0.47605906660965, 0.378943560134184, 0.341039687340247, 0.243183101838468, 
  0.210234902948755, 0.272947312948655, 0.285890513496704, 0.250377239119871, 
  0.293199920696763), station = c("1", "1", "1", "1", "1", "2", 
  "2", "2", "2", "2", "3", "3", "3", "3", "3", "4", "4", "4", "4", 
  "4", "5", "5", "5", "5", "5", "6", "6", "6", "6", "6", "1", "1", 
  "1", "1", "1", "3", "3", "3", "3", "3", "5", "5", "5", "5", "5", 
  "6", "6", "6", "6", "6", "1", "1", "1", "1", "1", "2", "2", "2", 
  "2", "2", "3", "3", "3", "3", "3", "4", "4", "4", "4", "4", "5", 
  "5", "5", "5", "5", "6", "6", "6", "6", "6", "1", "1", "1", "1", 
  "1", "3", "3", "3", "3", "3", "5", "5", "5", "5", "5", "6", "6", 
  "6", "6", "6", "1", "1", "1", "1", "1", "2", "2", "2", "2", "2", 
  "3", "3", "3", "3", "3", "4", "4", "4", "4", "4", "5", "5", "5", 
  "5", "5", "6", "6", "6", "6", "6", "1", "1", "1", "1", "1", "3", 
  "3", "3", "3", "3", "5", "5", "5", "5", "5", "6", "6", "6", "6", 
  "6")), .Names = c("PLI", "index_value", "station"), class = "data.frame", row.names = c(NA, 
  -150L))
  </code></pre>