Click on bar chart or legend to change which group moves to the y=0 baseline.
D3 version of Alan Dix's dancing histograms at http://www.meandeviation.com/dancing-histograms/
The problem
A stacked histogram allows three judgements: (i) the trends on the total height of the columns, (ii) the proportion of each category within each column and (iii) the trends in the lowest category. The trends, or even inter-column comparisons for any other category is very difficult as the blocks are at different heights.
The interactive stacked histogram solves this problem by allowing different trends to be analysed using the same dynamic graph. It is an example of a general princple of adding interactivity to existing paper visualisations.