plotting different level of classficifaction

hello, useRs~
suppose i have a matrix as follows:
item    category      sub-category
  A           1                  11
  B           1                  12
  C           1                  12
  D           2                  21
  E           2                  22

i like to draw a plot that represent the proportion of each
category at each level. that is, i want a plot like this:
    +-----+   +-----+  
    |        |   |   11 |
    |        |   +-----+
    |   1   |   |   12  |
    |        |   |        |
    +-----+   +-----+
    +-----+   +-----+
    |        |   |   21 |
    |   2   |   +-----+
    |        |   |   22 |
    +-----+   +-----+
  category   sub-cateory

hope my plot makes sense. anyone know how to make such plot?
any hint/suggestion would be much appreciate. thank you.