title: r gganimate (0.9.9.9) and transform_polygon for radar charts
body: <p>I'd like to use gganimate to transition through states in a radar chart.</p>
  
  <p>Consider the following example from a post here on Stack Overflow by Curycu :</p>
  
  <p><a href="https://stackoverflow.com/questions/28898143/closing-the-lines-in-a-ggplot2-radar-spider-chart">Closing the lines in a ggplot2 radar / spider chart</a></p>
  
  <pre><code>library(dplyr)
  library(data.table)
  library(ggplot2)
  
  rm(list=ls())
  
  scale_zero_to_one &lt;- 
    function(x) {
      r &lt;- range(x, na.rm = TRUE)
      min &lt;- r[1]
      max &lt;- r[2]
      (x - min) / (max - min)
    }
  
  scaled.data &lt;-
    mtcars %&gt;%
    lapply(scale_zero_to_one) %&gt;%
    as.data.frame %&gt;%
    mutate(car.name=rownames(mtcars)) 
  
  plot.data &lt;-
    scaled.data %&gt;%
    melt(id.vars='car.name') %&gt;%
    rbind(subset(., variable == names(scaled.data)[1]))
  
  # create new coord : inherit coord_polar
  coord_radar &lt;- 
    function(theta='x', start=0, direction=1){
      # input parameter sanity check
      match.arg(theta, c('x','y'))
  
      ggproto(
        NULL, CoordPolar, 
        theta=theta, r=ifelse(theta=='x','y','x'),
        start=start, direction=sign(direction),
        is_linear=function() TRUE)
    }
  
  plot.data %&gt;%
    ggplot(aes(x=variable, y=value, group=car.name, colour=car.name)) + 
    geom_polygon(aes(fill=car.name), alpha=0.2) +
    geom_point(size=rel(0.9)) +
    coord_radar() + 
    facet_wrap(~ car.name, nrow=4) + 
    theme_bw() +
    theme(
      axis.title.y = element_blank(),
      axis.text.y = element_blank(),
      axis.ticks.y = element_blank(),
      axis.title.x = element_blank(),
      legend.position = 'none') +
    labs(title = "Cars' Status")
  </code></pre>
  
  <p>This produces an amazing faceted plot for each car.  I made a slight modification to the original code and changed the <code>geom_path</code> to <code>geom_polygon</code>.  </p>
  
  <p>Instead of having a facetted graph, I would like to display the plot as a single chart that transitions through the <code>car.name</code>.  However, using <code>transition_states</code> produces an error:</p>
  
  <pre><code>plot.data %&gt;%
        ggplot(aes(x=variable, y=value, group=car.name, colour=car.name)) + 
        geom_polygon(aes(fill=car.name), alpha=0.2) +
        geom_point(size=rel(0.9)) +
        coord_radar() + 
        transition_states(car.name, transition_length = 3, state_length = 6) +
        enter_grow() +
        exit_shrink() +
        ease_aes('sine-in-out') +
        theme_bw() +
        theme(
          axis.title.y = element_blank(),
          axis.text.y = element_blank(),
          axis.ticks.y = element_blank(),
          axis.title.x = element_blank(),
          legend.position = 'none') +
        labs(title = "Cars' Status")
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Error in transform_polygon(all_frames, next_state, ease, params$transition_length[i],  : 
    transformr is required to tween polygons
  </code></pre>
  
  <p>The issue is not specific to geom_polygon, as replacing it with geom_path produces a similar error.  Is there a way to modify this to get it to work?</p>