title: ggplot2 geom_errorbar in reshaped data
body: <p>In the data.frame appended I would like to trend device sensitivity at 2 distances with measurement t date, facetted by monitor. This is pretty straightforward in <code>ggplot2</code>. First melt the data usinge the <code>reshape2</code> package</p>
  
  <pre><code>data.melt &lt;- melt(big_df[,c(1:3,6)],id=c("Date","Monitor"))
  </code></pre>
  
  <p>then use ggplot2 </p>
  
  <pre><code>ggplot(data.melt, aes(x=Date, y=value, col=variable)) + geom_point() + facet_wrap(~Monitor)
  </code></pre>
  
  <p>I would also like to add geom_errorbar to these points. The problem I have is the error bars differ for the 50cm and 100cm measurements and I am unsure how to define the limits. i.e if <code>variable = Sensitivity.100cm</code> set the limits to <code>value+/-Error.100cm</code> and for <code>variable = Sensitivity.50cm</code> <code>limits = value+/-Error.50cm</code>. How can I best approach this task?</p>
  
  <pre><code>structure(list(Date = c("18/10/2012", "19/10/2012", "22/10/2012", 
  "23/10/2012", "23/10/2012", "26/10/2012", "29/10/2012", "31/10/2012", 
  "11/01/2012", "11/02/2012", "11/06/2012", "11/08/2012", "11/09/2012", 
  "20/11/2012", "27/11/2012", "18/12/2012", "14/01/2012", "23/01/2013", 
  "18/01/2013", "16/02/2013", "23/04/2013", "30/04/2013", "07/05/2013", 
  "14/05/2013", "21/05/2013", "17/10/2012", "18/10/2012", "19/10/2012", 
  "22/10/2012", "23/10/2012", "24/10/2012", "26/10/2012", "29/10/2012", 
  "31/10/2012", "11/01/2012", "11/02/2012", "11/06/2012", "11/08/2012", 
  "11/09/2012", "20/11/2012", "27/11/2012", "18/12/2012", "14/01/2013", 
  "23/01/2013", "18/02/2013", "16/02/2013", "14/04/2013", "30/04/2013", 
  "07/05/2013", "14/05/2013", "21/05/2013", "16/10/2012", "18/10/2012", 
  "19/10/2012", "22/10/2012", "23/10/2012", "24/10/2012", "26/10/2012", 
  "29/10/2012", "31/10/2012", "31/10/2012", "01/11/2012", "02/11/2012", 
  "06/11/2012", "08/11/2012", "09/11/2012", "20/11/2012", "27/11/2012", 
  "18/12/2012", "14/01/2013", "23/01/2013", "18/02/2013", "16/04/2013", 
  "23/04/2013", "30/04/2013", "07/05/2013", "14/05/2013", "21/05/2013", 
  "16/04/2013", "23/04/2013", "30/04/2013", "07/05/2013", "14/05/2013", 
  "21/05/2013", "16/04/2013", "23/04/2013", "30/04/2013", "07/05/2013", 
  "14/05/2013", "21/05/2013"), Sensitivity.100cm = c(23.9310344827586, 
  23.6792452830189, 23.0708661417323, 23.75, 31.3333333333333, 
  25.1351351351351, 25.9770114942529, 24.5192307692308, 25.4347826086957, 
  22.987012987013, 23.1451612903226, 22.4822695035461, 23.4375, 
  23.9495798319328, 23.5245901639344, 24.041095890411, 25.4198473282443, 
  23.9097744360902, 25.8536585365854, 26.1940298507463, 28.0794701986755, 
  26.4041095890411, 24.3875968992248, 24.7019867549669, 26.4383561643836, 
  21.2328767123288, 23.5172413793103, 24.0566037735849, 30.7874015748032, 
  23.1111111111111, 29.5, 23.0405405405405, 26.2068965517241, 25, 
  24.3478260869565, 23.5064935064935, 22.0161290322581, 23.468085106383, 
  22.8125, 25.3781512605042, 22.2131147540984, 24.7945205479452, 
  23.6923076923077, 24.1353383458647, 25.2439024390244, 26.9402985074627, 
  27.9470198675497, 23.8835616438356, 25.3798449612403, 25.0331125827815, 
  25.7534246575342, 31.1627906976744, 30.6896551724138, 28.7735849056604, 
  29.7637795275591, 28.5294117647059, 40, 29.7972972972973, 33.448275862069, 
  33.4653465346535, 33.4653465346535, 36.9565217391304, 31.4285714285714, 
  30.8870967741935, 28.5106382978723, 29.0625, 29.4117647058824, 
  31.3934426229508, 33.5616438356164, 29.0151515151515, 30.8270676691729, 
  29.6341463414634, 34.3283582089552, 32.7152317880795, 37.2602739726027, 
  38.2945736434108, 35.8940397350993, 33.5616438356164, 40.5223880597015, 
  36.0264900662252, 33.8356164383562, 34.4186046511628, 36.158940397351, 
  33.7671232876712, 36.7910447761194, 36.0264900662252, 33.8356164383562, 
  30.5426356589147, 36.158940397351, 33.7671232876712), Sensitivity.50cm = c(89.448275862069, 
  89.4339622641509, 88.0314960629921, 88.4558823529412, 94.6666666666667, 
  85.9459459459459, 92.2988505747126, 93.6538461538461, 91.5217391304348, 
  88.2467532467532, 98.1451612903226, 85.6028368794326, 88.28125, 
  90, 85.655737704918, 87.7397260273973, 88.7786259541985, 90.8270676691729, 
  92.1341463414634, 89.6268656716418, 96.6887417218543, 91.1986301369863, 
  89.7364341085271, 87.0198675496689, 90.4794520547945, 80.8219178082192, 
  83.8620689655172, 85.188679245283, 82.992125984252, 88.1481481481482, 
  93.3333333333333, 87.9054054054054, 90.6896551724138, 89.3269230769231, 
  89.1304347826087, 90.1298701298701, 82.9032258064516, 82.6879432624114, 
  87.265625, 88.8235294117647, 87.7868852459016, 90.5479452054795, 
  91.7692307692308, 83.5338345864662, 92.0121951219512, 94.1044776119403, 
  88.0132450331126, 90.8013698630137, 89.7984496124031, 87.6158940397351, 
  88.2191780821918, 116.434108527132, 110, 116.509433962264, 112.44094488189, 
  110.147058823529, 123.333333333333, 107.5, 119.655172413793, 
  127.524752475248, 127.524752475248, 118.478260869565, 96.3636363636364, 
  112.338709677419, 105.815602836879, 116.5625, 116.806722689076, 
  118.27868852459, 129.452054794521, 113.106060606061, 115.789473684211, 
  115, 133.582089552239, 132.05298013245, 136.575342465753, 154.573643410853, 
  118.675496688742, 122.602739726027, 133.805970149254, 132.05298013245, 
  136.575342465753, 154.573643410853, 132.185430463576, 136.506849315069, 
  133.805970149254, 132.05298013245, 136.575342465753, 154.573643410853, 
  132.185430463576, 136.506849315069), Error.100cm = c(1.3139695781557, 
  1.56444565582802, 1.40192864683188, 1.36970117722038, 1.67497927018681, 
  1.33092672997245, 1.78068199825628, 1.60608587389328, 1.71862916313499, 
  1.29219147676184, 1.40378186980074, 1.29420479368047, 1.39754248593737, 
  1.46276430130498, 1.42679468733846, 1.31571045974648, 1.42197984810665, 
  1.39046932397796, 1.30208271366236, 1.4335352770372, 1.39230437361779, 
  1.37754694254238, 1.41544528946403, 1.3095178763765, 1.37669529056451, 
  1.81215843223602, 1.30671002217217, 1.60377358490566, 1.7304929902233, 
  1.34969386357693, 1.63724022536571, 1.28022265685787, 1.79545969561073, 
  1.60896158948861, 1.69788036432753, 1.30840530387077, 1.36621567307237, 
  1.32512183966048, 1.37108818553002, 1.49853399160731, 1.39344262295082, 
  1.33165904761805, 1.38033526499434, 1.40462719490747, 1.28772634645993, 
  1.45283002492028, 1.38599003552691, 1.31267619160706, 1.44319351564034, 
  1.32118790352715, 1.35955022200543, 1.5848874651365, 1.47108475915183, 
  1.72669860544558, 1.61753059717907, 1.49248405369003, 1.86338998124982, 
  1.44758684362355, 2.02050526795942, 1.88379184063767, 1.88379184063767, 
  2.06235499576199, 1.47504112877929, 1.60684345525575, 1.45000342555042, 
  1.53888403153064, 1.61641882871188, 1.63318515124355, 1.54679312200496, 
  1.50945900342207, 1.55913092882163, 1.38241268887252, 1.63499270897065, 
  1.48969826214596, 1.61504467432214, 1.74375533010884, 1.56437242530036, 
  1.54679312200496, 1.76125727182178, 1.56156637384789, 1.54071532591808, 
  1.65536096930718, 1.55875527098541, 1.54223702043804, 1.68034779838772, 
  1.56156637384789, 1.54071532591808, 1.56197222322557, 1.55875527098541, 
  1.54223702043804), Error.50cm = c(2.49898909406806, 2.94121835247545, 
  2.66090476357729, 2.57562939531426, 2.84312035153866, 2.42491330336843, 
  3.28542665010493, 3.03760942287141, 3.183873590408, 2.43051523015752, 
  2.83178223646443, 2.48024233333982, 2.64934764965831, 2.77310924369748, 
  2.66991761883645, 2.46860565629316, 2.61888806082572, 2.63908703380214, 
  2.39518528553687, 2.6055358582233, 2.54601096513127, 2.51706504452232, 
  2.65881302492679, 2.41699173389333, 2.50632526897698, 3.38331206444616, 
  2.42264387175866, 2.88777884245547, 2.66556076847414, 2.57667068606312, 
  2.82842712474619, 2.45392159423444, 3.26120941588761, 2.96210034629795, 
  3.15029929264988, 2.45725817252266, 2.60322780509177, 2.44047901652299, 
  2.62969260620419, 2.75266207632928, 2.70491803278689, 2.50538919974957, 
  2.67245427169849, 2.53752436551672, 2.39363249102323, 2.66888876749965, 
  2.42875772462149, 2.51128061073898, 2.6601687552902, 2.42695128500777, 
  2.47524808213598, 3.02027282771754, 2.76292767227726, 3.35536212146769, 
  3.02099623515762, 2.86858958869576, 3.22748612183951, 2.71029339461238, 
  3.74049197592482, 3.5862911438168, 3.5862911438168, 3.62137635846906, 
  2.52830160180932, 3.02499989250178, 2.75411175020533, 3.03382622483556, 
  3.15545938748212, 3.12874339081841, 2.99339361416605, 2.94091650663359, 
  2.96968686588444, 2.66763937246675, 3.1749324456989, 2.96611863265678, 
  3.06769803788475, 3.47196832194708, 2.81593266562507, 2.91397909399762, 
  3.17230019504959, 2.96611863265678, 3.06769803788475, 3.47196832194708, 
  2.96463964047674, 3.06846257323317, 3.17230019504959, 2.96611863265678, 
  3.06769803788475, 3.47196832194708, 2.96463964047674, 3.06846257323317
  ), Monitor = c("Berthold Red", "Berthold Red", "Berthold Red", 
  "Berthold Red", "Berthold Red", "Berthold Red", "Berthold Red", 
  "Berthold Red", "Berthold Red", "Berthold Red", "Berthold Red", 
  "Berthold Red", "Berthold Red", "Berthold Red", "Berthold Red", 
  "Berthold Red", "Berthold Red", "Berthold Red", "Berthold Red", 
  "Berthold Red", "Berthold Red", "Berthold Red", "Berthold Red", 
  "Berthold Red", "Berthold Red", "Berthold Blue", "Berthold Blue", 
  "Berthold Blue", "Berthold Blue", "Berthold Blue", "Berthold Blue", 
  "Berthold Blue", "Berthold Blue", "Berthold Blue", "Berthold Blue", 
  "Berthold Blue", "Berthold Blue", "Berthold Blue", "Berthold Blue", 
  "Berthold Blue", "Berthold Blue", "Berthold Blue", "Berthold Blue", 
  "Berthold Blue", "Berthold Blue", "Berthold Blue", "Berthold Blue", 
  "Berthold Blue", "Berthold Blue", "Berthold Blue", "Berthold Blue", 
  "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", 
  "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", 
  "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", 
  "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "NC 61", "Mini Red", 
  "Mini Red", "Mini Red", "Mini Red", "Mini Red", "Mini Red", "Mini Blue", 
  "Mini Blue", "Mini Blue", "Mini Blue", "Mini Blue", "Mini Blue"
  )), .Names = c("Date", "Sensitivity.100cm", "Sensitivity.50cm", 
  "Error.100cm", "Error.50cm", "Monitor"), class = "data.frame", row.names = c(NA, 
  90L))
  </code></pre>