I am looking for an alternative spreadsheet to Excel, preferably but not necessarily open source, that allows a programmer to create a plugin that can update cells in the sheet from an external data source in real time. The spreadsheet would then internally compute all dependent calculation chains upon change of value.
This is similar functionality to what the RTD method does with Microsoft Excel. The rate of external data change could be moderate to high (whatever such relativistic terms mean).
Also the reverse process would be useful, i.e. detecting a change in cells and then sending that information to a plugin that can communicate with external processes.
Any recommendations or experience in trying this?