Change displayed label of dimension of a barchart

[#1]

I want to display a timetable with the following data:

LOAD * INLINE[

Id, Begin, End, Group

L1|1, 08:00:00, 12:00:00, L1

L1|2, 12:45:00, 16:00:00, L1

L2|1, 15:00:00, 18:00:00, L2

]

As you can see, "Group" is part of the "Id". To create the timetable, i need a bar for each Id (which is my dimension), which has the length (End - Begin) as expression and an offset of "Begin". Now I intend to display "Group" as the dimension, but if I do so and add "Group" as a second dimension, nothing can be read as dimension and in addition something weird happens to the diagram itself.

This is the way it is displayed right now

This is how it looks like, if i add the second dimension (the column with Ids is empty and it is only one row displayed, i assume)

And that's the way i want the diagram to look like

Please notice that the whole dataset is about 6000 records

Can anyone help?