title: Plotting contours on an irregular grid
body: <p>I have gone through pages and pages of contour plots in R (including many hints on stackoverflow) without success. Here is my data to contour, including adding a map of Rwanda (the data consists of 14 values of longitude, latitude and rain as x,y and z): </p>
  
  <pre><code>Lon Lat Rain
  28.92   -2.47   83.4
  29.02   -2.68   144
  29.25   -1.67   134.7
  29.42   -2.07   174.9
  29.55   -1.58   151.5
  29.57   -2.48   224.1
  29.6    -1.5    254.3
  29.72   -2.18   173.9
  30.03   -1.95   154.8
  30.05   -1.6    152.2
  30.13   -1.97   126.2
  30.33   -1.3    98.5
  30.45   -1.81   145.5
  30.5    -2.15   151.3
  </code></pre>
  
  <p>Here is the code I tried from stackoverflow:</p>
  
  <pre><code>datr &lt;- read.table("Apr0130precip.txt",header=TRUE,sep=",")
  x &lt;- datr$x
  y &lt;- datr$y
  z &lt;- datr$z
  
  require(akima)
  
  fld &lt;- interp(x,y,z)
  
  par(mar=c(5,5,1,1))
  filled.contour(fld)
  </code></pre>
  
  <p>The interpolation fails.help will be appreciated.</p>