This paper introduces an interactive web platform called 'SLIDER' to explore longitudinal data and an original graphical display called 'slide plot' which is conceived to visualize aggregated trajectories. The paper begins with a short state of the art of existing graphical displays used to analyze longitudinal data. Then, it presents the main characteristics of the proposed slide plot visualization. At last, it gives a technical description of the web application and the graphical display, both implemented using the R software and the shiny R package.