title: Creating a heatmap based on historical map in R
body: <p>I have a data frame with several pre-1918 Russian districts ("Gouvernement") and the count of Waggons with relief aid going there:</p>
  
  <pre><code>df &lt;- structure(list(Gouvernement = structure(c(31L, 22L, 19L, 13L, 
  29L, 18L, 17L, 27L, 4L, 25L, 35L, 28L, 7L, 34L, 15L, 24L, 2L, 
  10L, 16L, 23L, 8L, 6L, 26L, 3L, 20L, 1L, 32L, 14L, 12L, 30L, 
  11L, 5L, 21L, 9L, 33L), .Label = c("Amur", "Astrachan", "Buchera", 
  "Charkow", "Donsches Heeresgebiet", "Fergana", "Irkutsk", "Jennisejsk", 
  "Kaluga", "Kasan", "Kostroma", "Minsk", "Moskau", "Nischnij Nowgorod", 
  "Olonetz", "Orel", "Orenburg", "Pensa ", "Perm", "Priamur", "Rjasan", 
  "Samara ", "Samarkand", "Saratow", "Semipalatinsk", "Simbirsk", 
  "Syr-Darja", "Tobolsk", "Tomsk", "Trans-Kaspische Provinz", "Transbaikalien", 
  "Turgai", "Twer ", "Ufa", "Wjatka"), class = "factor"), Waggons = c(118, 
  75, 56, 46, 46, 40, 34, 32, 30, 26, 24, 22, 22, 20, 19, 18, 13, 
  13, 13, 12, 12, 9, 7, 6, 6, 6, 5, 4, 3, 3, 1, 1, 1, 1, 1)),
  .Names = c("Gouvernement", "Waggons"), row.names = c(NA, -35L), class = "data.frame")
  </code></pre>
  
  <p>As you can see, there are some districts which received most of the Waggons, whereas others received only a small share:</p>
  
  <pre><code>ggplot(df, aes(x = Gouvernement, weight = Waggons)) +
          geom_bar() + 
          theme(axis.text.x = element_text(angle = 45, hjust = 1))
  </code></pre>
  
  <p>I want to print a heatmap which colorizes the districts based on the share of Waggons received. This heatmap should be based on the following historic map:
  <a href="http://content.staatsbibliothek-berlin.de/dms/PPN755491602/1200/0/00000053.jpg" rel="nofollow">Map of the Russian Empire 194-1918</a> (<a href="http://ngcs.staatsbibliothek-berlin.de/?action=metsImage&amp;format=jpg&amp;metsFile=PPN755491602&amp;divID=PHYS_0053&amp;original=true" rel="nofollow">TIFF with 21 Mb</a>)</p>
  
  <p>How can I digitize this map and use it for creating a heatmap with R?</p>
  
  <p>I use a Mac, OS X Yosemite, R version 3.2.2, and R Studio Version 0.99.441.</p>