[Issue] Adjust Legend for the Population Density View

<img width="1186" height="699" alt="Image" src="https://github.com/user-attachments/assets/a8b546ec-eadb-466d-93b7-e37a56bc9a31" />

Since the Mean and Median are represented by vertical lines rather than areas under curves, could we adjust the legend to represent these with colored lined, rather than the boxes used by the other elements?