I happened to see this thread and also wondering the answer to the chart drill down. Hope someone can point out a good way to this problem. With respect to bar chart series, I am also not quite familiar. Seems that I need to dig more deep into the chart.
So, drill down you are talking abut in real is behaviour, where you chart trying to present dimension level of your hierarchy group, where there are more than 1 dimension coordinates. Notice, that presented dimension level may change even if you select something outside the chart (on another chart or listbox).
My way to let user to have dimension level "under control" it was dynamic dimension. I loaded the island table with dimension names. User can select dimension and on that selection i turned on/off dimension in chart.