Live in situ visualization of numerical simulations - interactive visualization while the simulation is running - can enable new modes of interaction, including computational steering. Designing easy-to-use distributed in situ architectures, with viewing latency low enough, and frame rate high enough, for interactive use, is challenging. Here, we propose a fully asynchronous, hybrid CPU-GPU in situ architecture that emphasizes interactivity. We also present a transparent implementation of this architecture embedded into the OpenFPM simulation framework. The benchmarks show that our architecture minimizes visual latencies, and achieves frame rates between 6 and 60 frames/second - depending on simulation data size and degree of parallelism - by changing only a few lines of an existing simulation code.