Black Red Gradient

  size(400, 100);
  background(255); // white background
  
  float left_margin = 10;
  float top_margin = 10;
  float w = width - left_margin*2;
  float h = height - top_margin*2;
  
  for (int i = 0; i < h; ++i) {
    float ratio = i/h;
    float red = 256 * ratio;
    float green = 0;
    float blue = 0;
    stroke( red, green, blue );
    line(left_margin, top_margin+i, left_margin+w, top_margin+i);
  }