canvas ：曲線的面積圖 加漸變效果

document.body.innerHTML = '<canvas></canvas>'
var cvs = document.querySelector("canvas")
cvs.style.border = "1px solid red"
var ctx = cvs.getContext("2d");


var x0,y0;
for(var i=0;i<300;i++){
  var x = i,y=Math.sin(x/10)*10+50;
  ctx.strokeStyle="rgba(0,255,0,1)";
  ctx.beginPath();
  ctx.moveTo(x0||x,y0||y);
  ctx.lineTo(x,y);
  //ctx.stroke();

  x0 = x,y0=y;

  var linear = ctx.createLinearGradient(x,y,x,y+100);
  linear.addColorStop(0,"rgba(0,255,0,0.6)");
  linear.addColorStop(1,"rgba(255,255,255,0)");
  ctx.strokeStyle= linear; 

  ctx.beginPath();
  ctx.moveTo(x,y);
  ctx.lineTo(x,y+100);
  ctx.stroke();


  var linear = ctx.createLinearGradient(x,y+50,x,y+120);
  linear.addColorStop(0,"rgba(255,0,0,0.6)");
  linear.addColorStop(1,"rgba(255,255,255,0)");
  ctx.strokeStyle= linear; 

  ctx.beginPath();
  ctx.moveTo(x,y+50);
  ctx.lineTo(x,y+120);
  ctx.stroke();
}