How to group and aggregate in a view?

I have defined a model similar to the following:

   name: char

   group: many2one

   qty: float

   performance: function(float)

I would like to show view displaying group, qty and performance. But the group may be repeated for different records. So, I want to show every group only once, summing qty and performance (which is a function field).

I don´t what to show expandable groups. Just, one line per group.