title: Can't plot a scale bar or north arrow on ggplot2
body: <p>Help! I have a data set of GPS x/y locations for trees in a forest, and with this codes into a pretty map in ggplot: </p>
  
  <pre><code>#ggmap!
  library(ggmap)
  library(mapproj)
  map &lt;- get_map(location = 'Madagascar', zoom = 10)
  geocode("kianjavato")
  #lon       lat
  #47.86682 -21.38024
  k &lt;- "kianjavato"
  myMap &lt;- get_map(location=k, source="stamen", maptype="toner", crop=FALSE, zoom=16)
  ggmap(myMap)  
  m &lt;- ggmap(myMap) + geom_point(aes(y = Lat, x = Lon, colour = Foraged, shape=Plot), data = GPS) 
  n &lt;- m + scale_colour_manual(values=c("blue", "red3")) +
       scale_shape_discrete(solid=F, legend=F) +
       scale_y_continuous(limits=c(-21.376,-21.3715)) +
       scale_x_continuous(limits=c(47.865,47.869))
  plot(n)
  </code></pre>
  
  <p>However, no matter what types of code I try I can't get an arrow pointing north or scale bar to plot on my map! I've tried many solutions, some of which look like this: </p>
  
  <p>trying to add scale bar</p>
  
  <pre><code>map.scale &lt;- ggmap(new) + (ggmap, extent = "normal", maprange = FALSE) %+% sites.data +
      geom_point(aes(x = lon, y = lat, colour = colour)) + 
      geom_text(aes(x = lon, y = lat, label = label), hjust = 0, vjust = 0.5, size = 8/ptspermm) +
      geom_segment(data = sbar, aes(x = lon.start, xend = lon.end,
                                    y = lat.start, yend = lat.end)) + 
      geom_text(data = sbar, aes(x = (lon.start + lon.end)/2,
                                 y = lat.start + 0.025*(bb$ur.lat - bb$ll.lat),
                label = paste(format(distance, digits = 4, nsmall = 2),'km')),
                hjust = 0.5, vjust = 0, size = 8/ptspermm)  + 
      coord_map(projection="mercator", xlim=c(bb$ll.lon, bb$ur.lon), 
                ylim=c(bb$ll.lat, bb$ur.lat))  
  
  #library(SDMTools)
  #Scalebar(x=47.868,y=-21.375,distance=100,unit='m') #show values in meters
  #Error in map.scale(x = 50, y = -22) : argument "len" is missing, with no default
  
  map.scale(x=47.868, y=-21.375, ratio=FALSE, relwidth=0.2)
  #Error in map.scale(x = 47.868, y = -21.375, ratio = FALSE, relwidth = 0.2) : 
    unused argument(s) (ratio = FALSE, relwidth = 0.2)
  map.scale(x=47.868, y=-21.375)
  #Error in map.scale(x = 47.868, y = -21.375) : 
    argument "len" is missing, with no default
  </code></pre>
  
  <p>I need both the north arrow and the scale but neither will plot! Why?</p>