Sketching is a common practice among visualization designers and serves an approachable entry to data visualization for non-experts. However, moving from a sketch to a full fledged data visualization often requires throwing away the original sketch and recreating it from scratch. Our goal is to formalize these sketches, enabling 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. To demonstrate our proposed workflow, we implement DataGarden and evaluate it through a reproduction and a freeform study. We investigate how DataGarden supports personal expression and delve into the variety of visualizations that authors can produce with it, identifying cases that demonstrate the limitations of our approach and discussing avenues for future work.