LabVIEW has this, so you might check it out in LabVIEW Community if you're interested in this. Data flows across wires, and so you can probe the data in a running program to inspect it, which uses default displays for the probes. You can create custom probes for the data, which allows custom user interface interactions that takes in data from the probes (custom displays, additional calculations, etc.).
I am in general interested in this as well, and have found debugging in most text-based IDEs to be a step backwards.