Metric first-order temporal logic (MFOTL) is an expressive formalism for specifying temporal and data-dependent constraints on streams of time-stamped, data-carrying events. It serves as the specification language of several runtime monitors. These monitors input an MFOTL formula and an event stream prefix and output satisfying assignments to the formula’s free variables. For complex formulas, it may be unclear why a certain assignment is output. We propose an approach that accompanies assignments with detailed explanations, in the form of proof trees. We develop a new monitor that outputs such explanations. Our tool incorporates a formally verified checker that certifies the explanations and a visualization that allows users to interactively explore and understand the outputs.