title: ggplot2 incorrect median for a boxplot of identical values
body: <p>Among many thousands of data sets my program occasionally encounters
  those containing only identical values, for which I need ggplot2
  to plot the same as base R does: simply plot the median at its correct
  value.  Is there a way for ggplot2 to do this?</p>
  
  <pre><code>dput(df)
  
  structure(list(station = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 1L, 1L, 1L), .Label = "x", class = "factor"), value = c(1e-04, 
  1e-04, 1e-04, 1e-04, 1e-04, 1e-04, 1e-04, 1e-04, 1e-04, 1e-04
  )), .Names = c("station", "value"), class = "data.frame", row.names = c(NA, 
  -10L))
  
  station value
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  x   1.00E-04
  
  # via base R:
  boxplot(df$value ~ df$station,
  main="base R:  correct median, \ncorrect data range")
  
  # via ggplot2:
  library(ggplot2)
  ggplot(df, aes(factor(df$station), df$value)) + 
  geom_boxplot() +
  ggtitle("ggplot2:  incorrect median, \nincorrect data range")
  </code></pre>