Of course I've tried the drill down, its cool!
The documentation is enough.
Suggestion: When you drill down instead of making an ajax call allow to instantiate the plugin with all the data.

{
"COLUMNS":["X_BAR_LABEL","X_BAR_VALUE","X_BAR_COLOR", "TOOL_TIP_TITLE"],
"DATA":[
 ["Tom",1238,"#AA0000","Tom ^ ", 


{
            "COLUMNS":["X_BAR_LABEL","X_BAR_VALUE","X_BAR_COLOR", "TOOL_TIP_TITLE"],
            "DATA":[
             ["Jan",1238,"#AA0000","Jan ^ "],
 
            ["Feb",1069,"#AA0000","Feb ^ "],
             ["Mar",1214,"#AA0000","Mar ^ "],
             ["Apr",1262,"#AA0000","Apr ^ "],
             ["May",1975,"#AA0000","May ^ "],
             ["Jun",1076,"#AA0000","Jun ^ "]
                  ]
            }]},
 ["Dick",1069,"#AA0000","Dick ^ "],
 ["Harry",1214,"#AA0000","Harry ^ "],
 ["Mary",1262,"#AA0000","Mary ^ "],
 ["Louise",1975,"#AA0000","Louise ^ "],
 ["Sue",1076,"#AA0000","Sue ^ "]
]
}

See what I mean, all the data is there from the begining.
Suggestion #2: make the pluggin self contained, init it and nothing else no:
In the Drill down example there a lot of code outside the plugin and it confuses me, I don't know if all the code is necessary for the plugin to work or with

$("#chart_div").ddBarChart({ .......
will be enough

 If I have more ideas I'll will post them here :)
HTH