Stacked Barplot for tracing composition : I need to plot a set of data in a stacked barplot. The data are from six samples with three diferents compositions (M1, M1BIS, M2,M2BIS, M3 & M3BIS), and has been taken in three diferents points (surface(S), center(M) & bottom (G)) and two times (0 & 32 days).

I need to graph those data in a stacked barplot (one bar per sample and time) to see the salt concentration in the three points along time, and allde bars must have the same length (expressed in percentage)

Thanks a lot in advance

    rdata <-
structure(list(Sample = c("M1", "M1", "M1", "M2", "M2", "M2", 
"M3", "M3", "M3", "M1 BIS", "M1 BIS", "M1 BIS", "M2 BIS", "M2 BIS", 
"M2 BIS", "M3 BIS", "M3 BIS", "M3 BIS", "M1", "M1", "M1", "M2", 
"M2", "M2", "M3", "M3", "M3", "M1 BIS", "M1 BIS", "M1 BIS", "M2 BIS", 
"M2 BIS", "M2 BIS", "M3 BIS", "M3 BIS", "M3 BIS"), Composition = c(1L, 
1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L, 1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 
3L, 1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 3L, 1L, 1L, 1L, 2L, 2L, 2L, 
3L, 3L, 3L), t = c(0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
0L, 0L, 0L, 0L, 0L, 0L, 0L, 32L, 32L, 32L, 32L, 32L, 32L, 32L, 
32L, 32L, 32L, 32L, 32L, 32L, 32L, 32L, 32L, 32L, 32L), Value = c("1,72", 
"1,3", "4,12", "3,7", "2,88", "3,52", "1,38", "2,57", "6,89", 
"1,28", "0,85", "4,15", "0,71", "1,47", "4,77", "1,17", "1,37", 
"4,76", "4,16", "3,65", "3,39", "3,62", "4,04", "4,37", "2,77", 
"2,6", "3,23", "3,1", "3,34", "3,64", "3,68", "2,82", "3,12", 
"4,03", "3,87", "4,27"), Point = c("G", "M", "S", "G", "M", "S", 
"G", "M", "S", "G", "M", "S", "G", "M", "S", "G", "M", "S", "S", 
"M", "G", "S", "M", "G", "S", "M", "G", "S", "M", "G", "S", "M", 
"G", "S", "M", "G")), .Names = c("Sample", "Composition", "t", 
"Value", "Point"), row.names = c(NA, -36L), class = "data.frame")

