With the advent of the big data era, the significance of data analysis has increasingly come to the forefront, showcasing its ability to uncover valuable insights from vast datasets, thereby enhancing the decision-making process for users. Nonetheless, the data analysis workflow faces three dominant challenges: high coupling in the analysis workflow, a plethora of interactive interfaces, and a time-intensive exploratory analysis process. To address these challenges, this study introduces Navi, a data analysis system powered by natural language interaction. Navi embraces a modular design philosophy that abstracts three core functional modules from mainstream data analysis workflows: data querying, visualization generation, and visualization exploration. This approach effectively reduces the coupling of the system. Meanwhile, Navi leverages natural language as a unified interactive interface to seamlessly integrate various functional modules through a task scheduler, ensuring their effective collaboration. Moreover, in order to address the challenges of exponential search space and ambiguous user intent in visualization exploration, this study proposes an automated approach for visualization exploration based on Monte Carlo tree search. In addition, a pruning algorithm and a composite reward function, both incorporating visualization domain knowledge, are devised to enhance the search efficiency and result quality. Finally, this study validates the effectiveness of Navi through both quantitative experiments and user studies.