A Lindenmayer system that draws a spiral on a square tiling, starting from its center. See also a hexagonal one.
The X -> XF rule takes into account the growth in length of the spiral sides (one unit is added for each half turn), while the Y -> Y+XF+XF describes the concatenation of half turns of the spiral.
Modified http://d3js.org/d3.v3.min.js to a secure url
https://d3js.org/d3.v3.min.js