canvas绘制坐标轴

var canvas = document.getElementById("canvas"),
    context = canvas.getContext("2d"),

    AXIS_MARGIN = 40,    //一个常量
    AXIS_ORIGIN = {x:AXIS_MARGIN,y:canvas.height-AXIS_MARGIN},   //原点坐标

    AXIS_TOP = AXIS_MARGIN,        //纵轴端点
    AXIS_RIGHT = canvas.width-AXIS_MARGIN,//横轴端点

    HORIZONTAL_TICK_SPACING = 10,   //横轴间距
    VERTICAL_TICK_SPACING = 10,    //纵轴间距

    AXIS_WIDTH = AXIS_RIGHT-AXIS_ORIGIN.x,    //横轴长度
    AXIS_HEIGHT=AXIS_ORIGIN.y-AXIS_TOP,       //纵轴长度

    NUM_VERTICAL_TICKS = AXIS_HEIGHT/VERTICAL_TICK_SPACING,    //纵轴标尺的数量
    NUM_HORIZONTAL_TICKS = AXIS_WIDTH/HORIZONTAL_TICK_SPACING,  //横轴标尺的数量

    TICK_WIDTH = 10,
    TICKS_LINEWIDTH = 0.5,
    TICKS_COLOR = "navy",

    AXIS_LINEWIDTH = 1.0,
    AXIS_COLOR = "blue";

//一个函数,由于绘制网格
function drawGrid(context,color,stepx,stepy){
    context.strokeStyle = color;
    context.lineWidth = 0.5;

    for(var i = stepx + 0.5; i < context.canvas.width;i += stepx){
        context.beginPath();
        context.moveTo(i,0);
        context.lineTo(i,context.canvas.height);
        context.stroke();
    }

    for(var i = stepy + 0.5;i < context.canvas.height;i +=stepy){
        context.beginPath();
        context.moveTo(0,i);
        context.lineTo(context.canvas.width,i);
        context.stroke();
    }
}

function drawAxes(){
    context.save();
    context.strokeStyle = AXIS_COLOR;
    context.lineWidth = AXIS_LINEWIDTH;

    drawHorizontalAxis();
    drawVerticalAxis();

    context.lineWidth = 0.5;
    context.lineWidth = TICKS_LINEWIDTH;
    context.strokeStyle = TICKS_COLOR;

    drawHorizontalAxisTicks();
    drawVertialAxisTicks();
    drawNumberals();
}

//横坐标
function drawHorizontalAxis(){
    context.beginPath();
    context.moveTo(AXIS_ORIGIN.x,AXIS_ORIGIN.y);
    context.lineTo(AXIS_RIGHT,AXIS_ORIGIN.y);
    context.stroke();
}

//纵坐标
function drawVerticalAxis(){
    context.beginPath();
    context.moveTo(AXIS_ORIGIN.x,AXIS_ORIGIN.y);
    context.lineTo(AXIS_ORIGIN.x,AXIS_TOP);
    context.stroke();
}

//绘制纵坐标标尺及刻度数
function drawHorizontalAxisTicks(){
    var deltaY,num=0;

    for (var i = 1;i<NUM_HORIZONTAL_TICKS;++i){
        context.beginPath();
        if(i%5===0){
            deltaY = TICK_WIDTH;
            text();
            num++;
        }else {
            deltaY = TICK_WIDTH/2;
        }
        context.moveTo(AXIS_ORIGIN.x + i*HORIZONTAL_TICK_SPACING,AXIS_ORIGIN.y - deltaY);
        context.lineTo(AXIS_ORIGIN.x + i*HORIZONTAL_TICK_SPACING,AXIS_ORIGIN.y + deltaY);
        context.stroke();

        function text(){
            context.font = "12pt Helvetica";
            context.fillText(num,AXIS_ORIGIN.x +(i-6)*HORIZONTAL_TICK_SPACING,AXIS_ORIGIN.y + 3*deltaY);
        }
    }
}

//横坐标标尺及刻度
function drawVertialAxisTicks(){
    var deltaX,num=1;

    for (var i=1;i<NUM_VERTICAL_TICKS;++i){
        context.beginPath();
        if(i % 5 === 0){
            deltaX = TICK_WIDTH;
            text();
            num++;
        }else{
            deltaX = TICK_WIDTH/2;
        }
        context.moveTo(AXIS_ORIGIN.x - deltaX,AXIS_ORIGIN.y - i*VERTICAL_TICK_SPACING);
        context.lineTo(AXIS_ORIGIN.x + deltaX,AXIS_ORIGIN.y - i*VERTICAL_TICK_SPACING);
        context.stroke();

        function text(){
            context.font = "12pt Helvetica";
            context.fillText(num,AXIS_ORIGIN.x - 3*deltaX,AXIS_ORIGIN.y - i*VERTICAL_TICK_SPACING);
        }
    }
}

drawGrid(context,"lightgray",10,10);
drawAxes();