Plotting a semi circle

create a plot describing a semi-circle.

HTML

<!-- Graph layout with semi circle plotted -->  
<body>
    <canvas id="myCanvas" width="578" height="500"></canvas>
  </body>

CSS

body {
        margin: 0px;
        padding: 0px;
      }

JavaScript

var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');

var width = 500;
var height = 500;

drawGrid(width,height,'#ccc');
drawAxis(width,height,'#666');
// whole circle in grey
drawSemicircle(width/2, height/2, 100, 0, 360, 10, '#ccc');

function drawSemicircle(cx, cy, r, startAngle, endAngle, increment, style)
{
    var x=0;
    var y=0;
    for (a=startAngle;a<=endAngle;a+=increment) {
        x = r * Math.cos(deg2Rad(a)) + cx;
        y = r * Math.sin(deg2Rad(a)) + cy;
        plotPoint(x, y, 4, style);
    }
}

function plotPoint(x, y, size, style) {

    var radius = size/2;
    context.save();
    context.beginPath();
    context.translate(x, y);
    context.arc(0,0,radius,0,2 * Math.PI, true);
    context.fillStyle=style;
    context.fill();
    context.restore();
}


function drawAxis(w,h,style)
{
    // x axis
    var x = w/2;
    var y = h/2;
    context.fillStyle=style;
    for (var i=-x;i<x;i+=(w*.05)){
        context.fillText(i, x+i, y+12);
    }
    // y axis
    for (var i=-y;i<y;i+=(h*.05)){
        context.fillText(i, x-(context.measureText(i).width+4), y+i);
    }
}

function drawGrid(w,h,style)
{
    context.beginPath();
    var x=0;
    var y=0;
    for (y=0;y<h;y+=(h*.05)){
            context.moveTo(x, y);
            context.lineTo(w, y);
    }
    y=0;
    for (var x=0;x<w;x+=(w*.05)){
            context.moveTo(x, y);
            context.lineTo(x, h);
    }
    context.strokeStyle=style;
    context.stroke();
}

function deg2Rad(deg){
    return deg*Math.PI/180;
}