ggplot2 geom_bar: Centering bars

I would really appreciate help with a ggplot2 issue. I've been looking around it for long, so my best chance is now here. I provide a reproducible script.

What I'd need to do is center the bars horizontally. For now, they polarise towards the sides--especially notably on the 2009 and 2012 bars.

Centering them would allow me to freely adjust their width, whereas now I cannot.

This is the current, self-contained code:

install.packages('ggplot2')
library(ggplot2)

# Data    

Year = c('2009','2009','2009','2009','2010','2010','2010','2010','2011','2011','2011','2011','2012','2012','2012','2012')

City = c('Kaunas','Vilnius','Kaunas','Vilnius','Kaunas','Vilnius','Kaunas','Vilnius',
         'Kaunas','Vilnius','Kaunas','Vilnius','Kaunas','Vilnius','Kaunas','Vilnius')

Busy = c('Peak', 'Peak', 'Off-peak', 'Off-peak', 'Peak', 'Peak', 'Off-peak', 'Off-peak', 'Peak', 'Peak', 'Off-peak', 
         'Off-peak', 'Peak', 'Peak', 'Off-peak', 'Off-peak')

Result = c(-0.417277550641143, 0.540415934506096, -0.887323992801467, -1.0016151369497, -1.69512596882652, 
           -0.474180286913426, -1.87972395432173, -1.55647964460481, -1.10385766877877, -3.19261229467907, 
           -2.11137007243349, -3.24910554029829, -0.949278709972917, -2.22558030912235, -1.3496651478542, -2.59961942855099)

pvalue.asterisks = c(NA, NA, NA, NA, NA, NA, NA, NA, NA, '*', NA, '*', NA, NA, NA, NA)

Results = data.frame(Year, City, Busy, Result, pvalue.asterisks)

str(Results)
Results

# Plot

ggplot(Results, aes(Year, Result, fill=City)) +
  ylab(expression(paste(italic('t'), '-value'))) +
  facet_grid(list('Busy', 'Year')) + scale_fill_grey(start = 0, end = .6) +
  geom_bar(width=2.9, stat = "identity", position = position_dodge(width = 3)) +
  scale_y_continuous(expand = c(.035,0)) + scale_x_discrete(expand = c(.7,.4)) +
  theme( axis.title.x=element_blank(), axis.text.x = element_blank(),
         axis.title.y = element_text(size=19, face="bold", margin = margin(0, 3, 0, 0)),
         axis.text.y = element_text(size = 13),
         legend.title = element_blank(), legend.text = element_text(size=17),
         legend.background = element_rect(fill="white", size=0.1, linetype="solid"),
         legend.key.size = unit(1.7,"line"), axis.ticks.x=element_blank(), legend.position = c(.167, .6),
         panel.background = element_blank(),    plot.title = element_blank(),
         legend.margin=margin(c(-.05,.14,.11,.1), unit='cm'), 
         panel.grid.major.y = element_line(colour = "grey90"),
         panel.grid.major.x = element_blank(), plot.margin = unit(c(.05, .1, 0, .1), 'cm'),
         strip.text.x = element_text(face='bold', size=15, margin = margin(.2, 0, .2, 0, "cm")),
         strip.text.y = element_text(face='bold', size=15, margin = margin(0, .2, 0, .2, "cm")),
         strip.background = element_rect(colour="grey48", fill='grey95'), panel.spacing = unit(.08, "lines")) +
  geom_text(aes(x=Year, y=Result, label=pvalue.asterisks), vjust=-.01, colour= 'white', size=15, hjust=-.168 )

-- Thank you very much

If you remove the theme stuff you will see the problem, the X axis is years, and it's attempting to plot each one on the x-value for that year. One way to fix this is to set the scales parameter to facet_grid to be "free_x".

facet_grid(list('Busy', 'Year'), scales="free_x")

and then you only have to adjust the sizing of the bars.

geom_bar(width=0.5, stat = "identity", position = "dodge")