HTML5 Canvas (Math Issue)

[The Little Guy]

I am messing around with HTML5's canvas tag, I am currently building a graph, and the problem that I am having is this:

 

This code is within a function:

points = new Array();
points.push(10);
points.push(50);
points.push(10);
plotGraph(points);

function plotGraph(values){
ctx.strokeStyle = "rgb(0,0,0)";
ctx.lineWidth = 2;
ctx.beginPath();
ctx.moveTo(0, 0);
var max = values.max();
var ratio = max / ctx.canvas.height;
var x = 0;
for(var i in values){
	var val = values[i];
	var y = (ratio * val) / ctx.canvas.height;
	ctx.lineTo(x * 20, y);
	x++;
}
ctx.stroke();
}

 

What I need is for y to get calculated but in reverse (I assume). If you look at my array, 10 is a small value it should be at the bottom of the graph but it is near the top, where 50 is closer to the bottom of the graph. What would be a good way to reverse it?