  Building on previous developments, we show that the Diagrammatic Monte Carlo
technique allows to compute finite temperature response functions directly on
the real-frequency axis within any field-theoretical formulation of the
interacting fermion problem. There are no limitations on the type and nature of
the system's action or whether partial summation and self-consistent treatment
of certain diagram classes are used. In particular, by eliminating the need for
numerical analytic continuation from a Matsubara representation, our scheme
allows to study spectral densities of arbitrary complexity with controlled
accuracy in models with frequency-dependent effective interactions. For
illustrative purposes we consider the problem of the plasmon line-width in a
homogeneous electron gas (jellium).
