Using facet_grid and geom_hline together produces 2 empty extra panels

ggplot2_facet_hline_prob.r

#
# Packages
#
library(ggplot2)

#
# Data: 4 column dataframe.
# This is the first 10 rows of the dataframe but it makes the point
#
lvefeg <- structure(list(cvd_basestudy = c("CBP05J02", "CBP05J02", "CBP05J02",
 "CBP05J02", "CBP05J02", "CBP05J02", "CBP05J02", "CBP05J02", "CBP05J02",
 "CBP05J02"), ecd_rhythm = c("AF", "AF", "AF", "AF", "AF", "SINUS",
 "SINUS", "SINUS", "SINUS", "SINUS"), fixed_time = c(30.9, 33.17,
 32.57, 32.055, 30.9175, 30.5725, 33, 31.835, 34.045, 31.3975), 
     variable_time = c(29.4225, 32.035, 30.2775, 33.7275, 28.3475, 
     29.745, 31.155, 30.7, 33.48, 29.815)), .Names = c("cvd_basestudy", 
 "ecd_rhythm", "fixed_time", "variable_time"), class = c("cast_df",
 "data.frame"), row.names = c(1L, 2L, 3L, 4L, 5L, 6L, 7L, 9L,
 10L, 11L))

#
# Plot it:
# Looking to plot 2 panels showing the difference between variable
# and fixed time measures against the mean of the measures. The panels are 
# separated by the ecg_rhythm
#
# THE PROBLEM is that it produces 4 panels the top two are empty
# the bottom two are as expected

p <- qplot((variable_time + fixed_time) /2 , variable_time - fixed_time, data = lvefeg, geom='point') 
p + facet_grid(ecd_rhythm ~ .) + geom_hline(yintercept=0)


#
# The same result is obtained with:
# (Commented out here)
#
#g <- ggplot(lvefeg, aes(x=(variable_time + fixed_time)/2, y = variable_time - fixed_#time))
#g + geom_point() + facet_grid(ecd_rhythm ~ .) + geom_hline(yintercept=0)

#
# sessionInfo() output
#
#> sessionInfo()
#R version 2.12.1 (2010-12-16)
#Platform: i386-redhat-linux-gnu (32-bit)
#
#locale:
# [1] LC_CTYPE=en_GB.UTF-8       LC_NUMERIC=C              
# [3] LC_TIME=en_GB.UTF-8        LC_COLLATE=en_GB.UTF-8    
# [5] LC_MONETARY=C              LC_MESSAGES=en_GB.UTF-8   
# [7] LC_PAPER=en_GB.UTF-8       LC_NAME=C                 
# [9] LC_ADDRESS=C               LC_TELEPHONE=C            
#[11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C       
#
#attached base packages:
#[1] grid      stats     graphics  grDevices utils     datasets  methods  
#[8] base     
#
#other attached packages:
#[1] ggplot2_0.8.9 proto_0.3-8   reshape_0.8.3 plyr_1.4