[R] Barplot with "multiple categories"

[Noel Magnin]

Dear all
Thanks in advance for replies
I am trying to make a barplot out of this data which I read from a file

tb <- read.table("tmp.dat"
        , na.string=c("-"))

tmp.dat: ############
(the file is much longer and includes NAs as "-")

#    A1    A2    A3    B1    B2    B3    C1    C2    C3    D1    D2    D3
    2    P    12    2    P    11    2    F    1    2    P    7
    2    S    6    2    S    6    2    P    11    2    x    6
    2    x    34    2    x    21    2    S    2    3    I    7
    3    I    25    3    I    21    2    T    1    3    S    2
    3    N    2    3    S    1    2    x    15    3    x    4
    3    x    25    3    x    16    3    I    15    4    S    8
    4    C    1    4    S    26    3    x    15    4    Y    1
    4    S    32    4    x    12    4    S    15    4    x    4
    4    x    19    5    P    26    4    x    15    5    P    8
    5    L    1    5    R    2    5    P    15    5    S    1
    5    P    31    5    x    10    5    x    15    5    x    4
    5    R    1    6    I    27    6    I    13    6    I    9
    5    x    19    6    N    1    6    M    1    6    x    4
    6    I    32    6    x    10    6    N    1    7    E    9
    6    N    1    7    E    29    6    x    15    7    x    4
    6    x    19    7    x    9    7    D    1    8    T    9
    7    D    1    8    R    1    7    E    14    8    x    4
    7    E    32    8    T    29    7    x    15    9    V    10
    7    x    19    8    x    8    8    T    15    9    x    3
    8    T    34    9    V    30    8    x    15    10    P    11
    8    x    18    9    x    8    9    V    17    10    x    2
    9    I    1    10    H    1    9    x    13    11    V    11
    9    V    35    10    P    29    10    A    2    11    x    2
    9    x    16    10    x    8    10    P    15    12    K    13
    10    P    36    11    V    30    10    S    1    13    L    13
    10    x    16    11    x    8    10    x    12    14    K    13
    11    V    36    12    K    30    11    V    20    15    P    12
    11    x    16    12    R    1    11    x    10    15    [PA]    1
    12    K    41    12    x    7    12    K    23    16    G    13
    12    x    11    13    L    33    12    R    1    17    M    13
    13    I    2    13    x    5    12    x    6    18    D    13
    13    L    42    14    K    32    13    L    27    19    G    13
    13    x    8    14    N    1    13    x    3    20    P    13
    14    K    46    14    x    5    14    K    27    21    K    12

######### end Data
Data explanation :
"position" : A1, B1, C1, D1 for different samples  [these are 
amino-acids positions in biological sequences]
"value found" : A2, B2, ...
"number of occurences" : C1, C2 ....

the barplot type I would like to obtain :
 |
 |
y||
 || |
 || |
 |||| ...  ie : number of occurences (A3, B3, ...)
 |_______________________________________
  PSx PSx FPSTx Px | INx ISx Ix ISx ...   ie : values in A2, B2 ....(not 
always the same "length")
           2       |        3   ...           ie : position in A1, B1, ...

I have tried to read as matrix, simple vectors, but I cannot manage to 
reach this type of barplot.
Any hint towards this goal would be much appreciated
All the best
Noel