We propose an optical performance monitoring scheme that combines a tunable narrowband optical filter with asynchronous histograms to create density plots that represent probability maps of both optical power and frequency. Simulations are used to investigate the technique, and show that measurements at transmitters and filters can directly detect optical chirp. The effects of several optical impairments are also shown, comprising misaligned filters in the optical path, dispersion, nonlinear effects and optical signal to noise ratio. The density plots are affected differently by each of these impairments.