Granny squares are one of the most common Crochet techniques, and are often used in garment construction. Granny squares start with a circle, and four clusters of stitches are worked into it to create the corners. Each row makes the granny square bigger. One common approach to crochet garment construction involves making a lot of granny squares, in different sizes, and sewing them together.
Other shapes can be created in the same way, with additional corners. Granny hexagons (with six corners) are commonly used to make sweaters. A sweater (with voluminous sleeves, unless you do a lot of decreases at the end) can be made out of two large hexagons, potentially with some granny squares down the middle of the back if extra room is needed.
Granny squares are great for data visualization because they have a predictable number of clusters of stitches, and you can use those to represent data directly (e.g. one piece of data = one stitch), or as a proportion (30% of the stitches are one color/texture). If you’re representing data directly, you may need a “filler color” if your data doesn’t line up perfectly with the number of stitches (like the gray in How My Girlfriend’s Mother Introduces Me to Her Friends).
There are many different patterns and designs for granny squares.