Great tutorial! One issue I ran into was with the Gantt Position. The first question's Gantt position were correct however the following questions' positions began at weird intervals and as a result, the bars for each response were spaced out instead of being connected. I think this was caused by the nature of the PREVIOUS_VALUE statement. It carries on the value of the previous row, even it was from a different question.
To fix this, start by changing the the Gantt Position calculated field to: IF FIRST() = 0 THEN [Gantt Start] ELSE PREVIOUS_VALUE([Gantt Start])+ZN(lookup([Percentage],-1)) END
Then go into "Default Table Calculation" and change the "Compute Using" to "Response". This will make sure the questions are partitioned correctly and the Gantt Position will reset with every question. Hope this helps!