The process of verifying linear model assumptions and remedying associated violations is complex, even when dealing with simple linear regression. This process is not well supported by current tools and remains time-consuming, tedious, and error-prone. We present RegLine, a visual analytics tool supporting the iterative process of assumption verification and violation remedy for simple linear regression models. To identify the best possible model, RegLine helps novices perform data transformations, deal with extreme data points, analyze residuals, validate models by their assumptions, and compare and relate models visually. A qualitative user study indicates that these features of RegLine support the exploratory and refinement process of model building, even for those with little statistical expertise. These findings may guide visualization designs on how interactive visualizations can facilitate refining and validating more complex models.