title: World map with ggmap
body: <p>I am using ggmap and wish to have a map of the world centered on Australia to which I can easily plot geocoded points. ggmap seems to be a lot easier to use compared to some of the other mapping packages. Yet when I bring through a map using the code below it errors. </p>
  
  <pre><code>gc &lt;- geocode('australia')
  center &lt;- as.numeric(gc) 
  &gt; map &lt;- get_map(location = center, source="google", maptype="terrain", zoom=0)
  Error: zoom must be a whole number between 1 and 21
  </code></pre>
  
  <p>From the get_map help:
  "zoom: map zoom, an integer from <strong>0 (whole world) to 21</strong> (building), default value 10 (city). openstreetmaps limits a zoom of 18, and the limit on stamen maps depends on the maptype. 'auto' automatically determines the zoom for bounding box specifications, and is defaulted to 10 with center/zoom specifications."</p>
  
  <p>Changing the zoom to one does not error for get_map but does for plotting that map</p>
  
  <pre><code>map &lt;- get_map(location = center, source="google", maptype="terrain", zoom=1)
  ggmap(map)
  
  Warning messages:
  1: In min(x) : no non-missing arguments to min; returning Inf
  2: In max(x) : no non-missing arguments to max; returning -Inf
  3: In min(x) : no non-missing arguments to min; returning Inf
  4: In max(x) : no non-missing arguments to max; returning -Inf
  </code></pre>
  
  <p>It looks like the longitude isn't being pulled through. 
  Finally with a zoom of 2 it does work but does not bring through a map of the whole world</p>
  
  <p>So, my question is how can I use get_map to get a world map?</p>
  
  <p>Session info:</p>
  
  <blockquote>
    <p>sessionInfo()
        R version 2.15.0 (2012-03-30)
        Platform: i386-pc-mingw32/i386 (32-bit)</p>
  </blockquote>
  
  <pre><code>locale:
  [1] LC_COLLATE=English_United Kingdom.1252 
  [2] LC_CTYPE=English_United Kingdom.1252   
  [3] LC_MONETARY=English_United Kingdom.1252
  [4] LC_NUMERIC=C                           
  [5] LC_TIME=English_United Kingdom.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] mapproj_1.1-8.3 maps_2.2-6      rgdal_0.7-12    sp_0.9-99      
  [5] ggmap_2.1       ggplot2_0.9.1  
  
  loaded via a namespace (and not attached):
  [1] colorspace_1.1-1   dichromat_1.2-4    digest_0.5.2       grid_2.15.0       
  [5] labeling_0.1       lattice_0.20-6     MASS_7.3-17        memoise_0.1       
  [9] munsell_0.3        plyr_1.7.1         png_0.1-4          proto_0.3-9.2     
  [13] RColorBrewer_1.0-5 reshape2_1.2.1     RgoogleMaps_1.2.0  rjson_0.2.8       
  [17] scales_0.2.1       stringr_0.6        tools_2.15.0 
  </code></pre>