Q:

How can I rotate a linear gradient?

How can I make my white line go perfectly diagonal from point to point of my bottom rectangle?  
https://jsfiddle.net/a7rs5qu5/ 
  <canvas id="canvas" width="300" height="300"></canvas>

_canvas = document.getElementById('canvas');
_stage = _canvas.getContext('2d');

_stage.fillStyle = "#00FF00";
_stage.fillRect(0, 0, 300, 200);

var gradient = _stage.createLinearGradient(0, 200, 300, 300);
gradient.addColorStop(0, "blue");
gradient.addColorStop(.5, "white");
gradient.addColorStop(1, "blue");

_stage.fillStyle = gradient;
_stage.fillRect(0, 200, 300, 300);

A:

By mathematics, and changing gradient coordinates. You need to set gradient coordinates so they describe a line orthogonal to it.

_canvas = document.getElementById('canvas');
_stage = _canvas.getContext('2d');

_stage.fillStyle = "#00FF00";
_stage.fillRect(0, 0, 300, 200);

var radius = 100;
var angle = Math.atan2(100, 300) + Math.PI / 2;
var gx = radius * Math.cos(angle);
var gy = radius * Math.sin(angle);
var cx = (0 + 300) / 2;
var cy = (200 + 300) / 2;

var gradient = _stage.createLinearGradient(cx - gx, cy - gy, cx + gx, cy + gy);
gradient.addColorStop(0, "blue");
gradient.addColorStop(.5, "white");
gradient.addColorStop(1, "blue");

_stage.fillStyle = gradient;
_stage.fillRect(0, 200, 300, 300);
<canvas id="canvas" width="300" height="300"></canvas>

The selection of radius controls how wide the gradient is; the above value merely gives values similar to the ones used in the code above.

