r - ggplot2 binwidth with factor(variable)? -

I have a simple histogram, but binwidth can not be adjusted:

  Qplot (factor size (size), data = mydf, geom = "histogram", binwidth = 0.01)  

The size-class is a clear variable (four) that group another size variable I want to get all the following:

  plot (table (mydf $ size_class)  

using ggplot2. Code works but ignores any kind of bin-wide. It seems that this is something to do with my group variables, because examples of numerical variables work. How can I do this with variables

thx Advance for any help .. .

You can not set binwidth for a clear variable - this does not mean because there Not an inherent continuous scale.