title: Convert SpatialPointsDataFrame to SpatialLinesDataFrame in R
body: <p>I'm working with the HURDAT dataset to plot hurricane tracks.
  I have currently produced a SpatialPointsDataFrame object in R which looks something like this for the year 2004.</p>
  
  <pre><code>    &gt; str(cluster.2004.sdf)
  Formal class 'SpatialPointsDataFrame' [package "sp"] with 5 slots
    ..@ data       :'data.frame': 2693 obs. of  4 variables:
    .. ..$ Sid      : int [1:2693] 1331 1331 1331 1331 1331 1331 1331 1331 1331 1331 ...
    .. ..$ clusterid: num [1:2693] 2 2 2 2 2 2 2 2 2 2 ...
    .. ..$ name     : Factor w/ 269 levels "","ABBY      ",..: 6 6 6 6 6 6 6 6 6 6 ...
    .. ..$ WmaxS    : num [1:2693] 78.9 82.8 80.9 70.9 76.9 ...
    ..@ coords.nrs : num(0) 
    ..@ coords     : num [1:2693, 1:2] 754377 612852 684956 991386 819565 ...
    .. ..- attr(*, "dimnames")=List of 2
    .. .. ..$ : NULL
    .. .. ..$ : chr [1:2] "lon" "lat"
    ..@ bbox       : num [1:2, 1:2] -3195788 1362537 4495870 9082812
    .. ..- attr(*, "dimnames")=List of 2
    .. .. ..$ : chr [1:2] "lon" "lat"
    .. .. ..$ : chr [1:2] "min" "max"
    ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
    .. .. ..@ projargs: chr "+proj=lcc +lat_1=60 +lat_2=30 +lon_0=-60 +ellps=WGS84"
  
      &gt; summary(cluster.2004.sdf)
  Object of class SpatialPointsDataFrame
  Coordinates:
           min     max
  lon -3195788 4495870
  lat  1362537 9082812
  Is projected: TRUE 
  proj4string :
  [+proj=lcc +lat_1=60 +lat_2=30 +lon_0=-60 +ellps=WGS84]
  Number of points: 2693
  Data attributes:
        Sid         clusterid             name         WmaxS       
   Min.   :1331   Min.   :1.000   IVAN      :517   Min.   : 14.83  
   1st Qu.:1334   1st Qu.:2.000   FRANCES   :403   1st Qu.: 31.35  
   Median :1337   Median :3.000   JEANNE    :379   Median : 50.04  
   Mean   :1337   Mean   :2.898   KARL      :283   Mean   : 61.66  
   3rd Qu.:1339   3rd Qu.:4.000   DANIELLE  :271   3rd Qu.: 90.40  
   Max.   :1341   Max.   :4.000   BONNIE    :253   Max.   :142.52  
                                  (Other)   :587 
  </code></pre>
  
  <p>Each storm has a unique storm id reference labelled "Sid".
  I'd like to group the SpatialPointsDataFrame by the "Sid" and convert all the points into a Line.</p>
  
  <p>I've had a go with ddply from the plyr package but frankly have no idea what I'm doing.
  I know I can do this by looping round each row in the data frame and appending coordinates to a list, then converting that list using the Lines function from sp package.</p>
  
  <p>However, I'd rather a more R way of converting.
  Thanks
  Richard</p>