title: Changing the radius of a coord_polar ggplot
body: <p>I'm looking to reduce the display radius of a pie chart built in ggplot2 relative to the rest of the plot (as the defaults keep cutting off my category labels).</p>
  
  <p>Here's some dummy data and code that should show you what I'm experiencing:</p>
  
  <pre><code>library(ggplot2)
  library(scales)
  library(grid)
  
  Region &lt;- c("North America", "Central America", "South America", "Carribbean",
              "Western Africa", "Northern Africa", "Southern Afica", "Eastern Africa")
  Conti &lt;- c(rep("Americas",4), rep("Africa",4))
  Freq &lt;- c(runif(8, 1, 100))
  Pct &lt;- c(Freq/sum(Freq))
  Pos &lt;- c(cumsum(360*Pct)-(360*Pct/2))
  Pos &lt;- c(ifelse(Pos&lt;=180,Pos,Pos-180))
  df &lt;- data.frame(Region, Conti, Freq, Pct, Pos)
  
  pl &lt;- ggplot(df, aes(x="", y=Freq, fill=Conti)) +
    geom_bar(stat="identity", color="black", width=1) +
    coord_polar(theta='y') +
    guides(fill=guide_legend(override.aes=list(colour=NA))) +
    theme(axis.line = element_blank(),
          axis.ticks=element_blank(),
          axis.title=element_blank(),
          axis.text.y=element_blank(),
          axis.text.x=element_text(color='black', size=18, angle=90-df$Pos),
          panel.background = element_blank(),
          panel.border = element_blank(),
          panel.grid.major = element_blank(),
          panel.grid.minor = element_blank(),
          panel.margin = unit(0, "lines"),
          plot.background = element_rect(fill = "white"),
          plot.margin = unit(c(0, 0, 0, 0), "cm"),
          legend.position = "none") +
    scale_y_continuous(
      breaks=cumsum(df$Freq) - df$Freq/2,
      labels=paste0(df$Region," ",percent(df$Pct)))
  
  print(pl)
  </code></pre>
  
  <p>If I reduce the size of the labels they become illegible relative to the pie and if I increase them they get cut-off.  No matter how I try to adjust the aes(), limits, panel.margin etc. to get the balance right ggplot2 keeps automatically re-sizing the pie to occupy the same radii.</p>
  
  <p>Ideally, I'd like to shrink the pie by half to allow more room for labels.</p>
  
  <p>I appreciate that this isn't the prettiest plot, however, I'm updating an old figure so need to maintain the format for comparison. Any suggestions would be appreciated.</p>