Public space/furniture are amongst the new domains to apply a data-driven approach of design intervention and improvements. Open space is essentially dynamic, livable and interactive. Various types of people spend time for various purposes. Therefore, the "Measure-Test-Refine" loop is applicable for improving open spaces gradually. In this research, we developed our original smart chair called "Proto-Chair" that can contribute to the new design method of public space. Our chair is made with 3D-printed soft auxetic patterns. It is morphable allowing users to sit in various ways. Also, our chair is equipped with two sensors, which collect data stream to distinguish four different states of the chair. Long-term sensor stream can be stored and used to refine the furniture. In this paper, we propose our concept, prototypes, sensing methods and results of experiments. We also introduce our future vision of a sensor-based public design platform.