code>, <code>add_lines()</code>, etc) that define how to render data into geometric objects. Borrowing terminology from the layered grammar of graphics, these functions add a graphical layer to a plot. A <em>layer</em> can be thought of as a group of graphical elements that can be sufficiently described using only 5 components: data, aesthetic mappings (e.g., assigning <code>clarity</code> to <code>color</code>), a geometric representation (e.g. rectangles, circles, etc), statistical transformations (e.g.,