Uneven x-axis spacing for violinplots/boxplots

[mtaub...@gmail.com]

Is there a way to specify the locations along the x-axis for the violins or boxes for data grouped by a factor? Equivalent to the "at" argument in the standard boxplot function, e.g., if I am plotting 11 groups and I want more space between box 1 and box 2 than between the remaining boxes, I could do 

boxplot(x~y, at =c(0, seq(1.5,10.5, by=1)))

How could I do something like this for box plots or violin plots in ggplot2?

Cheers,

Margaret