RainbowBanner displays a smooth vertical gradient of color across the canvas, with each row shifting hue from left to right. The code iterates over every pixel coordinate using nested `d3.range` loops, computing a color via `d3.hcl` based on the pixel’s position. It renders directly to a Canvas 2D context using `fillRect` for each 1×1 pixel, leveraging D3 v4’s HCL color space and the `d3.hsv` and `d3-scale-chromatic` helper modules. No external data is used; the visualization is purely generative.