Question

I’m creating a canvas via javascript on a webproject.

The canvas has graphical representations on the x-y plane.

I am trying to add the horizontal-scrolling feature to the canvas.

I have investigated a few methodologies:-

1) draw 12 months worth of data on the canvas, when the mouse scrolls forward, 1st month’s data disappears, and at the end a new months data is added, new canvas is drawn.

Con:- Everytime the mouse scrolls to pan thru the timeline – a new SQL query has to be made, making my web application very slow.

2) maybe I can draw say 10 years worth of data on the canvas via 1 SQL query, but only show 12 months worth of the data. masking the rest of the 9 years. Now when the client scrolls, I capture the scroll event and move to the appropriate part of the canvas. Is this possible? If so then how?

Can anyone advise?

My current representation of the canvas = with only 12 months worth of data

To be more specific on the scrolling, I would like to have a feeling such as the following widget for my client-side scrolling action:-

http://www.simile-widgets.org/timeline/

Answer 1

Here’s a pretty basic implementation: http://jsfiddle.net/CQPeU/

var can = document.getElementById("can"),
    ctx = can.getContext('2d'),
    dragging = false,
    lastX = 0,
    translated = 0;

// these two lines will make the y-axis grow upwards. 
ctx.scale(1,-1);  
ctx.translate(0, -400);

can.onmousedown = function(e){
  var evt = e || event;
  dragging = true;
  lastX = evt.offsetX;
}

window.onmousemove = function(e){
  var evt = e || event;
  if (dragging){
    var delta = evt.offsetX - lastX;
    translated += delta;
    ctx.translate(delta, 0);  // translate the context.
    lastX = evt.offsetX;
    draw();  // redraw
  }
}

window.onmouseup = function(){
  dragging = false;
}


function draw() {
  ctx.clearRect(-translated, 0, 600, 400); // this is why we need to keep track of how much we've translated
  for (var i = 0; i < plot.length; i++) {
    ctx.beginPath();
    ctx.arc(plot[i].x, plot[i].y, 5, 0, 2 * Math.PI); // note we don't have to futz with the x/y values, and can use them directly. 
    ctx.fill();
  }
}

To create a grid, you could do something like this:

var grid = (function(dX, dY){
  var can = document.createElement("canvas"),
      ctx = can.getContext('2d');
  can.width = dX;
  can.height = dY;
  // fill canvas color
  ctx.fillStyle = 'black';
  ctx.fillRect(0, 0, dX, dY);

  // x axis
  ctx.strokeStyle = 'orange';
  ctx.moveTo(.5, 0.5);
  ctx.lineTo(dX + .5, 0.5);
  ctx.stroke();

  // y axis
  ctx.moveTo(.5, .5);
  ctx.lineTo(.5, dY + .5);
  ctx.stroke();

  return ctx.createPattern(can, 'repeat');
})(100, 50);

Which would be used like this:

function draw() {
  ctx.clearRect(-translated, 0, 600, 400);
  ctx.rect(-translated, 0, 600, 400);
  ctx.fillStyle = grid;
  ctx.fill();
  ctx.fillStyle = "#fff";
  for (var i = 0; i < plot.length; i++) {
    ctx.beginPath();
    ctx.arc(plot[i].x, plot[i].y, 5, 0, 2 * Math.PI);
    ctx.fill();
  }
}

Updated demo: http://jsfiddle.net/CQPeU/2/