var strum = 10;

​

function setup() { 

  createCanvas(1200, 50);

} 

​

function draw() { 

  background("#FFE1DC");

  stroke(1);

  noFill();

  beginShape();

  vertex(0, 100);

  for(var x = 0; x < width; x++){

    //var angle = map(x, 0, width, 0, TWO_PI);

    var angle = offset + x * 0.01;

    // map x between 0 and width to 0 and Two Pi

    var y = map(sin(angle), -strum, strum,0, 50);

    vertex(x, y);

  }

  vertex(width, height);

  endShape();

  offset += 0.01;

}