Customise/scale axis tick marks for 4 plots independent of each other in facet_wrap (RStudio) : [1]: http://i.stack.imgur.com/0Iq01.png

I have created the above plot [1] using the code below:

    ggplot(data = gdt, aes(x = area)) + 
    geom_histogram(bins = 10, colour = "black", fill = "grey50") +
    facet_wrap( ~ fires, scales = "free") +
    labs(x = "Area of Burnt Land (ha)", y = "Fires") +
    ggtitle("Count of Destructive Fires in Portugal (2007)")

I want change the interval occurrence or the location of the tick marks individually for each sub plot, in other words I am trying to achieve a placement of one tick/grid-line at each point the bars meet. I have tried using scale_continuous and scale_x_discrete to no effect, if I set the marks to suit one of the sub plots it negatively affects the others. Is there a way to do this?? 


