Stacked bar chart: In stacked bar chart the bars representing the sub-groups are placed on top of each other to make a single column (single bar). The overall length of the bar gives us the total size of the category whilst different colors helps us in indicating the relative contribution of the different sub-groups. The stacking of the bars means there is no common baseline for the individual bar elements where it becomes tedious for us to compare the subcategories.
From the below graph we infer that it is very difficult to compare the iron content of three different samples and we see that the the sum of all the dependent variable values for each bar is same