Sketching is a common practice among visualization designers, and an approachable entry to visualizations for individuals, but moving from a sketch to a full fledged data visualization often requires throwing away the original sketch recreating it from scratch. We aim to instead formalize these sketches to enable them to support iteration and systematic data mapping through a visual-first templating workflow. In this workflow, authors sketch a representative visualization and structure it into an expressive template for an envisioned or partial dataset, capturing implicit style as well as explicit data mappings. In order to demonstrate and evaluate our proposed workflow, we implement DataGarden, and evaluate it through a reproduction and a freeform study. We discuss how DataGarden supports personal expression, and delve into the variety of visualizations that authors can produce with it, identify cases which demonstrate the limitations of our approach and discuss avenues for future work.