I have the data for a lot of forwarding starting interest rate swaps. i.e 2Y1Y, 3Y1Y, 5Y1Y, 3Y2Y, 5Y2Y, ... (so different forwarding and maturities).
I would like to calculate the roll down over 1 year for each of them. I don't have other data (Libor, Euribor, etc). What I call a "roll-down" is the difference between xYzY - (x-n)YzY given that the yield curve stays the same. n is the roll-down period. For example, for the 2Y1Y, to get the one-year roll-down I do 2Y1Y - (2-1)Y1Y.
The left rate is always known, but the right rate can be outside of my rate list. So, I need to find its value.
From QuantLib, how could I retrieve this swap rate from all my input data and/or explain the process?
Thanks in advance.