CSS Animate image reveal

Question

I have a graph that I would like to animate using CSS if possible. The way I see myself doing it would be to use a mask so it starts with nothing and then slowly the mask goes across gradually revealing the graph line.

This isn't the image I'm using but will be similar: http://jsfiddle.net/e50mnm2p/

So imagine it starts off with nothing and then from the left it will gradually reveal the green line giving the impression it's being drawn. Is there a way of doing this with masks or is there a better alternative?

<img src="http://www.freeimages.com/assets/183018/1830173156/graph-line-up-and-down-1-1131299-m.jpg" />

UPDATE

This is what I ended up doing: http://jsfiddle.net/e50mnm2p/5/

HTML

<div class="hgInner">
    <img src="http://www.freeimages.com/assets/183018/1830173156/graph-line-up-and-down-1-1131299-m.jpg" />
</div>

CSS

.hgInner {
    position: relative;
    float: left;
    height: 100%;
    width: 955px;
    overflow: hidden;
    -webkit-animation: reveal 4s ease;
    -webkit-animation-fill-mode: backwards;
}

@-webkit-keyframes reveal {
    0%   {width: 0px;}
    100% { width: 100%;}
}
@keyframes reveal {
    0%   {width: 0px;}
    100% { width: 100%;}
}

Solution 1

Do you want something like this? http://jsfiddle.net/e50mnm2p/3/

.reveal {
    overflow:hidden;
    position:relative;
    display:inline-block;
}

.reveal:after {
    content:" ";
    position:absolute;
    display:block;
    top:0;
    left:0;
    width:100%;
    height:100%;
    background:#fff;
    z-index:2;
    transition: all 2s ease;
}

.reveal.show:after {
    left:100%
}

Solution 2

You can use a pseudo-element to hide the actual element. To reveal the element, animate the pseudo-element's transform property (rotate it by 90deg).

The HTML :

<div id="parent">
    <div id="child"></div>
</div>

The CSS :

#parent {
    overflow: hidden;
}

#child {
    position: relative;
    background-color: yellow;
}

/* Pseudo-element that hides the child */
#child::after {
    content: ''; 
    position: absolute;
    width: 150%;
    height: 150%;
    background-color: blue;
    top: 0;
    left: 0;
    bottom: 0;
    transform: rotate(0deg);
    transform-origin: 0px 0px;
    transition: transform linear 0.7s;
}

#child.animated::after {
    transform: rotate(90deg);
}

The Javascript :

$("#reveal-button").on('click', function() {
    $("#child").toggleClass('animated');
});

There's the second way to reveal hidden element explained in my blog post : http://usefulangle.com/post/37/revealing-hidden-elements-by-css-animations