  Lov\'asz's bound to the capacity of a graph and the the sphere-packing bound
to the probability of error in channel coding are given a unified presentation
as information radii of the Csisz\'ar type using the R{\'e}nyi divergence in
the classical-quantum setting. This brings together two results in coding
theory that are usually considered as being of a very different nature, one
being a "combinatorial" result and the other being "probabilistic". In the
context of quantum information theory, this difference disappears.
