title: R ggplot2 discrete colour palette for gradient map
body: <p>With the help from SO users I'm now able to plot a gradient map with <code>geom_tile</code> and <code>scale_fill_gradient</code>. Now I need to prescribe a fixed color palette. My first attempt was to use <code>scale_fill_manual</code> but I get this error message:</p>
  
  <blockquote>
    <p>Error: Continuous value supplied to discrete scale</p>
  </blockquote>
  
  <p>This is the code for the original plot</p>
  
  <pre><code>ggplot() + 
    geom_tile(data = idw.output, alpha = 0.8, aes(x = longitud, y = latitud, fill = RGlobal)) +
    scale_fill_gradient(low = "cyan", high = "orange",name = "UVI") +
    coord_map(xlim = c(-1.7, -1),ylim = c(37.7,38)) +
    ggtitle("Previsión UVI") + xlab(" ") + ylab(" ")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/bI92N.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bI92N.png" alt="enter image description here"></a></p>
  
  <p>The required palette is</p>
  
  <pre><code>paleta &lt;- c("#4eb400","#a0ce00","#f7e400","#f8b600","#f88700","#f85900","#e82c0e","#d8001d","#ff0099","#b54cff","#998cff")
  </code></pre>
  
  <p>Maybe I need to reformat data to produce a new binned data column (seen in some SO posts) that can be used with scale_fill_manual but I couldn't succeed</p>
  
  <p>This are the data used for the minimal example in the plot.</p>
  
  <pre><code>dput(idw.output)
  structure(list(longitud = c(-2, -1.9, -1.8, -1.7, -1.6, -1.5,
  -1.4, -1.3, -1.2, -1.1, -1, -0.9, -0.8, -0.7, -0.6, -0.5, -0.4,
  -0.3, -0.2, -0.0999999999999999, 0, -2, -1.9, -1.8, -1.7, -1.6,
  -1.5, -1.4, -1.3, -1.2, -1.1, -1, -0.9, -0.8, -0.7, -0.6, -0.5,
  -0.4, -0.3, -0.2, -0.0999999999999999, 0, -2, -1.9, -1.8, -1.7,
  -1.6, -1.5, -1.4, -1.3, -1.2, -1.1, -1, -0.9, -0.8, -0.7, -0.6,
  -0.5, -0.4, -0.3, -0.2, -0.0999999999999999, 0, -2, -1.9, -1.8,
  -1.7, -1.6, -1.5, -1.4, -1.3, -1.2, -1.1, -1, -0.9, -0.8, -0.7,
  -0.6, -0.5, -0.4, -0.3, -0.2, -0.0999999999999999, 0), latitud = c(37.7,
  37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7,
  37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.7, 37.8, 37.8,
  37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.8,
  37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.8, 37.9, 37.9, 37.9,
  37.9, 37.9, 37.9, 37.9, 37.9, 37.9, 37.9, 37.9, 37.9, 37.9, 37.9,
  37.9, 37.9, 37.9, 37.9, 37.9, 37.9, 37.9, 38, 38, 38, 38, 38,
  38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38
  ), RGlobal = c(2.47603407457515, 2.56722902275175, 2.65722090029236,
  2.74600970719686, 2.83359544346532, 2.91997810909774, 3.0051577040941,
  3.0891342284544, 3.17190768217865, 3.25347806526685, 3.333845377719,
  3.4130096195351, 3.49097079071514, 3.56772889125912, 3.64328392116706,
  3.71763588043894, 3.79078476907477, 3.86273058707454, 3.93347333443827,
  4.00301301116594, 4.07134961725755, 3.02937209763967, 3.08883053029498,
  3.14708589231421, 3.20413818369741, 3.25998740444456, 3.31463355455564,
  3.36807663403068, 3.42031664286968, 3.47135358107261, 3.52118744863947,
  3.56981824557031, 3.61724597186509, 3.6634706275238, 3.70849221254646,
  3.75231072693307, 3.79492617068364, 3.83633854379815, 3.8765478462766,
  3.915554078119, 3.95335723932535, 3.98995732989565, 3.54364142893506,
  3.57136334606903, 3.59788219256697, 3.62319796842884, 3.64731067365467,
  3.67022030824443, 3.69192687219814, 3.71243036551582, 3.73173078819742,
  3.74982814024299, 3.76672242165248, 3.78241363242593, 3.79690177256333,
  3.81018684206467, 3.82226884092997, 3.83314776915921, 3.84282362675239,
  3.85129641370953, 3.85856613003061, 3.86463277571564, 3.86949635076461,
  4.01884206846156, 4.01482747007424, 4.00960980105084, 4.00318906139137,
  3.99556525109589, 3.98673837016434, 3.97670841859673, 3.96547539639308,
  3.95303930355338, 3.93940014007761, 3.9245579059658, 3.90851260121793,
  3.891264225834, 3.87281277981403, 3.853158263158, 3.83230067586593,
  3.81024001793779, 3.78697628937361, 3.76250949017337, 3.73683962033708,
  3.70996667986473), var1.var = c(1.01843285546171, 0.862794784692161,
  0.732770319338128, 0.625346340387164, 0.537698048758539, 0.467188965298011,
  0.411370930784953, 0.367984105924389, 0.334956971355076, 0.310406327642763,
  0.292637295283383, 0.280143314704528, 0.271606146260796, 0.265895870237954,
  0.262070886851961, 0.259377916247975, 0.257251998500358, 0.255316493614272,
  0.253383081523816, 0.251451762093071, 0.249710855116251, 0.873286128828167,
  0.727373752540586, 0.606635566366428, 0.508058451395788, 0.428817608648967,
  0.366276559075455, 0.317987143554231, 0.281689522894669, 0.25531217783545,
  0.236971909045211, 0.22497383712203, 0.217811402594311, 0.214166365919716,
  0.212908807485795, 0.213097127609912, 0.213978046539185, 0.214986604450304,
  0.215746161449985, 0.216068397574453, 0.215953312789854, 0.215589226991921,
  0.887450912328089, 0.741743618032266, 0.621122629179308, 0.522574826962587,
  0.443275412504286, 0.380587906854426, 0.332064150996824, 0.295444305840244,
  0.268656852226255, 0.249818590925231, 0.237234642637044, 0.229398447992332,
  0.224991767550493, 0.222884681801266, 0.222135591163608, 0.221991215986408,
  0.221886596548835, 0.221445093059246, 0.220478385655308, 0.2189864744063,
  0.217157679308732, 0.913284629008054, 0.764062048323634, 0.640189419045726,
  0.538653622469394, 0.456629859817726, 0.391481652243769, 0.340760840830826,
  0.302207586592512, 0.273750370470586, 0.253505993338174, 0.239779575997525,
  0.231064559179595, 0.226042703546666, 0.223584089690392, 0.222747118131327,
  0.222778509320894, 0.223113303639578, 0.223374861397818, 0.223374862835642,
  0.223113308123219, 0.222778517360005)), .Names = c("longitud",
  "latitud", "RGlobal", "var1.var"), row.names = c("1", "2", "3",
  "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15",
  "16", "17", "18", "19", "20", "21", "22", "23", "24", "25", "26",
  "27", "28", "29", "30", "31", "32", "33", "34", "35", "36", "37",
  "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48",
  "49", "50", "51", "52", "53", "54", "55", "56", "57", "58", "59",
  "60", "61", "62", "63", "64", "65", "66", "67", "68", "69", "70",
  "71", "72", "73", "74", "75", "76", "77", "78", "79", "80", "81",
  "82", "83", "84"), class = "data.frame")
  </code></pre>
  
  <p>Thanks in advance for your help</p>