Final answer:
By default, a heat map uses rectangles to visualize data, as this shape efficiently fills up the grid space to convey information.
Step-by-step explanation:
In the context of data visualization, a heat map is a graphical representation of data where the individual values are represented as colors. When it comes to the shapes used by a heat map, the default shape is usually a rectangle. Heat maps are used in various fields such as meteorology, biology, and web analytics, and they display data in a matrix where rows and columns correspond to categories, and the color intensity reflects the magnitude of a particular value. The choice of using rectangles generally allows for an efficient way to fill up space within a grid and convey information quickly to the user.