title: Error: Discrete value supplied to continuous scale issue
body: <p>I am trying to plot a heat Map in R using ggmap. In this regard, i have followed this example. </p>
  
  <p><a href="https://stackoverflow.com/questions/18285415/density2d-plot-using-another-variable-for-the-fill-similar-to-geom-tile/">Density2d Plot using another variable for the fill (similar to geom_tile)?</a></p>
  
  <p>But when i execute the final line . i.e. </p>
  
  <pre><code>print(pred.stat.map.final) 
  </code></pre>
  
  <p>in the "Setting the colorMap" section , I get the following error</p>
  
  <pre><code>Error: Discrete value supplied to continuous scale
  </code></pre>
  
  <p>Here's the Data : </p>
  
  <p>Link to data for KAN.csv : 
  <a href="https://www.dropbox.com/s/ainm2gjewll7xk0/KAN.csv" rel="nofollow noreferrer">https://www.dropbox.com/s/ainm2gjewll7xk0/KAN.csv</a></p>
  
  <pre><code>StationName  | Total PrecipitationCount | Latitude | Longitude
  
  19.8 SE COLDWATER KS US 5626    37.0571 -99.09
  ABBYVILLE 1.1 SSE KS US 21587   37.9556 -98.197
  ABILENE 0.7 E KS US 12526   38.9197 -97.2095
  ABILENE 1.1 NE KS US    7284    38.9314 -97.2073
  ABILENE 11.9 N KS US    20222   39.09   -97.1823
  ABILENE 12.7 S KS US    9232    38.7383 -97.261
  ABILENE 2.4 WSW KS US   19335   38.9117 -97.2655
  ABILENE 3.2 ENE KS US   15648   38.9393 -97.1672
  ABILENE 5.4 E KS US 7431    38.9341 -97.1238
  ABILENE KS US   16493   38.9266 -97.2129
  ABILENE KS US   39004   38.9266 -97.213
  ABILENE KS US   1196    38.92667    -97.21306
  AGENDA KS US    42809   39.7083 -97.4311
  AGENDA KS US    13711   39.7134 -97.4516
  ALBERT 5 SE KS US   -18998  38.3899 -98.9504
  ALBERT 5 SE KS US   25149   38.39   -98.95028
  ALEXANDER KS US -1006972    38.4694 -99.552
  ALMA 6.1 NW KS US   36538   39.0674 -96.38
  ALTA VISTA KS US    43886   38.8619 -96.4725
  ALTA VISTA KS US    16418   38.862  -96.4892
  ALTAMONT KS US  73968   37.1944 -95.2875
  ALTAMONT KS US  8160    37.1952 -95.3002
  ALTON 10.0 S KS US  2344    39.3228 -98.9537
  ALTON 2 SW KS US    7696    39.44806    -98.97
  ALTON 2 SW KS US    3198    39.4483 -98.9702
  ALTON KS US -267345 39.47667    -98.97139
  ANDALE 0.4 SSE KS US    0   37.7873 -97.6257
  ANDALE 6.4 WSW KS US    46338   37.7636 -97.7399
  ANTHONY 0.4 NNW KS US   14754   37.1587 -98.0315
  ANTHONY KS US   26363   37.155  -98.0282
  ANTHONY KS US   29615   37.155  -98.02833
  ANTONINO 4.5 WSW KS US  24322   38.7555 -99.4579
  ANTONINO 7.5 W KS US    149 38.7845 -99.5194
  ARGONIA KS US   924 37.26167    -97.76889
  ARGONIA KS US   -11948  37.2618 -97.769
  ARKANSAS CITY 2.0 SSE KS US 13471   37.0431 -97.0239
  ARKANSAS CITY 3.1 N KS US   17606   37.1142 -97.0385
  ARKANSAS CITY 3.7 S KS US   15199   37.0146 -97.0428
  ARKANSAS CITY 3.9 SSW KS US 6219    37.0141 -97.0615
  ARKANSAS CITY KS US 58326   37.0631 -97.0399
  ARLINGTON 1.7 ESE KS US 26159   37.8859 -98.15
  ARLINGTON 5.5 SSE KS US 36858   37.8186 -98.1545
  ARLINGTON KS US 5496    37.8966 -98.2711
  ARLINGTON KS US -15003  37.89667    -98.27111
  ARLINGTON KS US -22910  37.933  -98.25
  ARNOLD 4.8 NNW KS US    38970   38.6956 -100.0664
  ASHLAND 14.6 SSE KS US  30038   37.0002 -99.6467
  ASHLAND 7.5 SE KS US    22135   37.111  -99.6717
  ASHLAND KS US   34626   37.1941 -99.7632
  ASSARIA 4.3 SE KS US    1544    38.6454 -97.5381
  ATCHISON 1 S KS US  10820   39.54833    -95.11806
  ATCHISON KS US  60155   39.5755 -95.1108
  ATLANTA KS US   -32405  37.4372 -96.7669
  ATLANTA KS US   -4907   37.43722    -96.76694
  ATWOOD 0.5 SSW KS US    18164   39.8028 -101.043
  ATWOOD 1.5 E KS US  10248   39.8126 -101.0137
  ATWOOD 11 N KS US   12839   39.9761 -101.0579
  ATWOOD 8 SSE KS US  28815   39.69778    -100.97083
  ATWOOD 8 SSE KS US  4162    39.698  -101.0039
  ATWOOD 8 SSE KS US  -380    39.69806    -101.00389
  ATWOOD KS US    -30204  39.7996 -101.0421
  ATWOOD KS US    -91615  39.8086 -101.0777
  AUBURN KS US    7900    38.9019 -95.8167
  AUBURN KS US    -141001 38.9088 -95.815
  AUBURN KS US    19261   38.9269 -95.8144
  AUGUSTA 0.8 NW KS US    5373    37.6994 -96.9861
  AUGUSTA 1.0 S KS US 28996   37.6782 -96.9723
  AUGUSTA KS US   63270   37.6983 -96.9705
  AURORA 7.7 SW KS US 14317   39.3595 -97.6105
  AXTELL KS US    -497939 39.86778    -96.25444
  AXTELL KS US    -1229101    39.8678 -96.2547
  BAILEYVILLE KS US   7905    39.83083    -96.18306
  BAILEYVILLE KS US   20899   39.8311 -96.1833
  BARNARD 1 S KS US   247 39.1774 -98.0413
  BARNARD 1 S KS US   44871   39.1886 -98.0468
  BARNARD 1 S KS US   -6909   39.1897 -98.0419
  BARNARD 7 W KS US   49393   39.2144 -98.1766
  BARNARD 7.5 ENE KS US   36571   39.2224 -97.9108
  BARNES 0.2 N KS US  52743   39.7144 -96.8729
  BARTLETT KS US  -8728   37.0485 -95.2142
  BARTLETT KS US  -74696  37.0485 -95.2145
  BAZINE 0.4 SSW KS US    11284   38.4405 -99.6944
  BEARDSLEY 1 N KS US 4346    39.8245 -101.23
  BEARDSLY 7.7 S KS US    14305   39.6981 -101.2252
  BEATTIE 2 NNW KS US 15095   39.89639    -96.425
  BEATTIE 2 NNW KS US 17701   39.8966 -96.4247
  BEATTIE 2 NNW KS US 9008    39.8966 -96.425
  BEAUMONT 0.1 SW KS US   8556    37.658  -96.533
  BEAUMONT 4.9 SSE KS US  36775   37.59   -96.51
  BEAUMONT KS US  7100    37.65556    -96.53528
  BEAUMONT KS US  -153940 37.6556 -96.5353
  BEAUMONT KS US  2967    37.6557 -96.5352
  BEELER 0.1 ENE KS US    4736    38.4446 -100.1952
  BELLE PLAINE 4 W KS US  -69130  37.393  -97.3497
  BELLEFONT 3 S KS US 10447   37.8263 -99.6542
  BELLEVILLE KS US    -43995  39.81833    -97.6375
  BELLEVILLE KS US    -78074  39.8186 -97.6377
      BELLEVILLE KS US    -131545 39.8233 -97.6386
      BELOIT 6.4 SW KS US 10212   39.4073 -98.2007
      BELOIT 8.1 ESE KS US    19016   39.407  -97.977
  </code></pre>
  
  <p>Here's my code : </p>
  
  <ol>
  <li><p>Reading my File .</p>
  
  <pre><code>csvdata_Kansas  = read.csv(file ="KAN.csv")
  </code></pre></li>
  <li><p>Using sqldf() to get reduced Subset .</p>
  
  <pre><code>StationList_Precipitation  = sqldf("Select distinct(STATION_NAME) as Station_Name, 
                             sum(PRCP) as TotalPrecipitationCount, LATITUDE as Latitude, LONGITUDE as Longitude from csvdata_Kansas where LATITUDE &lt;&gt; 'unknown' and LONGITUDE &lt;&gt; 'unknown'
                             group by STATION_NAME, LATITUDE, LONGITUDE")
  </code></pre></li>
  <li><p>I get following kind of result . </p>
  
  <pre><code> head(StationList_Precipitation)
  </code></pre></li>
  </ol>
  
  <p><img src="https://i.stack.imgur.com/DGjKi.jpg" alt="enter image description here"></p>
  
  <ol start="4">
  <li><p>I plot map . </p>
  
  <pre><code>map.in &lt;- get_map(location = c(lon = -94, lat = 42),
            source = c("osm"))
  ggmap(map.in)
  </code></pre></li>
  <li><p>Map Shows up . <img src="https://i.stack.imgur.com/itPpi.png" alt="enter image description here"></p></li>
  <li><p>I then execute this code , to get a heat map on the above map . </p>
  
  <pre><code>colormap &lt;- c("Violet","Blue","Green","Yellow","Red","White")
  head(StationList_Precipitation)
  pred.stat.map.final &lt;- ggmap(map.in) %+% StationList_Precipitation +
  aes(x  =Longitude,
  y = Latitude,
  z = TotalPrecipitationCount) +
  stat_summary2d(fun = median, 
           binwidth = c(.05, .05),
           alpha = 1.0) + 
   scale_fill_gradientn(name = "Median",
                 colours = colormap,
                 space = "Lab") + 
  labs(x = "Longitude",
  y = "Latitude") +
  coord_map()
  print(pred.stat.map.final)
  </code></pre></li>
  <li><p>I get error in the last line i.e. </p>
  
  <pre><code>print(pred.stat.map.final)
  </code></pre>
  
  <p>The error is : </p>
  
  <pre><code>Error: Discrete value supplied to continuous scale
  </code></pre></li>
  </ol>
  
  <p>What can be the problem?  </p>