int h = 200;   // hieght
int w = 200;   // width
int yGap = 10; // the gap (on Y axis) to craete between each line.
int yPos = 0;  // The variable that will define y position for each line.

int counter = 0;      // a counter variable for our loop  
int loopNum = h/yGap; // math for getting the number of lines to draw ..
                      // .. depending on the hight of our stage and the gap between each line

int redVal = 256;  // value to use in red component of the Stroke() command
int redGap = 256/loopNum;  // add this value for each new line.

size(w,h);
strokeWeight(1);

while(counter < 16){
  if(counter>4 && counter<13){
    strokeWeight(counter-1);
  }
  if(counter>12 && counter<16){
    yGap = counter*2;
    strokeWeight(counter*2);
  }
  stroke(redVal,0,0);
  redVal -= redGap;
  line(0,yPos, w,yPos);
  yPos += yGap; // same as: ypos = ypos + yGap -->  set yPos to its own value plus yGap --> increase yPos by yGap 
  counter ++;
}