set div width via javascript onDomReady

I'm using mootools 1.2 in a page where I'm loading a js graph into a div 开发者_StackOverflow中文版that is itself in a table cell. The graph loads properly, but it's quite a lot smaller than the width of the cell (the graph script is meant to generate a graph the same size as the div).

My question is: how can you set the size of the div based on the size of its parent container, dynamically (rather than setting a fixed pixel width of the div).

I'd like to set the size of the div with mootools setStyle on domready, like this (all wrapped in a domready block):

var divwidth    = $('my_chart').offsetParent.offsetWidth;
var divheight   = $('my_chart').offsetParent.offsetHeight;
$('my_chart').setStyle('width', divwidth);
$('my_chart').setStyle('height', divheight);

g           = new Dygraph($('my_chart'), data);

When I run the page without the setStyle stuff, it works, but the div returns a width of 480 when I execute $('my_chart').getStyle('width'); from a js console. However, the $('my_chart').offsetParent.offsetWidth from the same console returns 1100.

The problem is that when I try to set the div size using the code above, it throws an error: Uncaught TypeError: Cannot read property 'offsetWidth' of null I assume because the div is not drawn yet?

Edit to add: if I set the width of the my_chart div to "auto", I get an error Uncaught TypeError: Cannot read property 'length' of undefined from the graph script.

Thanks for any help.

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http://jsfiddle.net/dimitar/c7GTA/

jsfiddle right back at you.

the problem is that this tool uses canvas. and it kind if needs to set the canvas width to a set value.

var c = document.id("mychart");
c.setStyle("width", c.getParent().getSize().x);

new Dygraph(document.id("mychart"),
"Date,Temperature\n" + "2008-05-07,75\n" + "2008-05-08,70\n" + "2008-05-09,80\n"
);

this works fine but table resize will make it look odd. use .getSize() which returns an object with x and y properties for width and height. don't trust computations based upon css/style.