title: ggmap with coordinates with curved arrows between cities in R
body: <p>I would like to make a map of the Netherlands with curved lines between cities. I have two dataframes number one called: <code>df_verticles</code> which contains 24 cities with their lat/lon combination. The second dataframe called: <code>df</code> i want to use to draw a curved line between lat/lon from combination to the lat/lon to combination.</p>
  
  <pre><code>&gt; head(df_vertices)
          city AmountSessions   totalkWh AmountRFID scaledAmount   scaledkWh Latitude Longitude
  1    Alkmaar          13608  104554.68       1326   0.07139012 0.026941910 52.63903  4.755538
  2     Almere          11281  100841.42        930   0.05006999 0.025985067 52.39447  5.282043
  3 Amersfoort           7719   67663.30       1198   0.06449876 0.017435647 52.15108  5.383069
  4 Amstelveen          25794  236437.93       2616   0.14084204 0.060925915 52.31724  4.859266
  5  Amsterdam         402365 3880744.86      18574   1.00000000 1.000000000 52.34560  4.808834
  
  
  &gt; head(df)
    CityChargeSessions   NextCity Amount    sumkWh scaledAmount  scaledkWh Latitude_from Longitude_from Latitude_to Longitude_to
  1          Amsterdam    Alkmaar   1058  8133.736   0.18438480 0.15480933      52.34560       4.808834    52.63903     4.755538
  2          Amsterdam     Almere    998  7254.133   0.17392820 0.13806786      52.34560       4.808834    52.39447     5.282043
  3          Amsterdam Amersfoort    566  4977.404   0.09864064 0.09473489      52.34560       4.808834    52.15108     5.383069
  4          Amsterdam Amstelveen   3724 24210.289   0.64900662 0.46079423      52.34560       4.808834    52.31724     4.859266
  5             Almere  Amsterdam   1034  8685.526   0.18020216 0.16531155      52.39447       5.282043    52.34560     4.808834
  6         Amersfoort  Amsterdam    579  4936.823   0.10090624 0.09396251      52.15108       5.383069    52.34560     4.808834
  
  map &lt;- get_map(location = "Schiphol", zoom = 9, source = "google")
  p1 &lt;- ggmap(map)+
     geom_point(aes(x = Longitude, y = Latitude, size=scaledkWh), data = df_vertices, alpha =0.5)+
     scale_size_continuous(range=c(1,30)) + 
     geom_segment(data=df, aes(x=Longitude_from, y=Latitude_from, xend=Longitude_to, yend=Latitude_to),
             arrow=arrow(ends="first"), size= df$scaledAmount,alpha=0.75)
  </code></pre>
  
  <p>This is my output so far:</p>
  
  <p><img src="https://i.stack.imgur.com/oGCMk.png" alt="enter image description here"></p>
  
  <p>As you can see in this image above, some lines overlap which is not nice. Could anyone help me how to code this to curved lines between the cities.</p>
  
  <p>Thank you very much!</p>