I could definately be wrong on this but...
My understanding of chart drill down is really it is only acting the exact same as drilling down using a group name or group summary field in a GH or GF.
You can really ignore the chart all together to start with.
Try to understand your report from the group and the drill down using groups and there structure only first. Once you get how drilling into each group header works you can relate the group header to the bar in the chart that it represents. That bar will drill down the same as the group you drilled into inside the report.
Here is an example to help explain.
When using 2 levels of charts, you are not really drilling from one chart into another chart. You are drilling from the report header where your first chart was placed, into a group header where your second charts was placed so it looks like your drilling into another chart.