r - How to force the x-axis tick marks to appear at the end of bar in heatmap graph?

Question

I created a simple heatmap graph with ggplot2 but I need to force the x-axis tick marks to appear at the end of my x variable, rather than at its center. For example, I would expect 1 to appear at the position of where 1.5 is now. I beleive a heatmap done in Base R would do that.

library(car) #initialize libraries
library(ggplot2)  #initialize libraries
library(reshape)

df=read.table(text= "x  y  fill
1 1 B
2 1 A
3 1 B
1 2 A
2 2 C
3 2 A
",  header=TRUE, sep=""  )

#plot data
qplot(x=x, y=y, 
      fill=fill, 
      data=df, 
      geom="tile")+  
      scale_x_continuous(breaks=seq(1:3) ) 

The idea is to create a simple heatmap which looks like this:

The tick marks in this graph are placed at the end of the bars instead of their centers

Answer 1

What about this?

object = qplot(x=x, y=y, 
      fill=fill, 
      data=df, 
      geom="tile")+  
      scale_x_continuous(breaks=seq(1:3))

object + scale_x_continuous(breaks=seq(.5,3.5,1), labels=0:3)