Resize the fixed width of html elements to allow responsive design for e.g. d3 visualisations

responsive.js

/*

description:
  This function allows you to resize an html element
  when the screen size is changing (resize the browser or ).
  This allows you responsive design on some elements with
  fixed pixel sizes. This can be used e.g. for d3.js visualisations.

author: Florian von Bracht
email: f@dataflo.de

parameters:
  objID: The id of the html element.
  margin: optional. You can define a margin around the element like {top: 20, right: 20, bottom: 20, left: 40}.

usage:
  If you have an element like this
  
  <div id="d3-line"></div>
  
  you can resize it like in the following code. When the whole document was loaded, the element size
  will be set and then everytime when the resolution changes.
  
  // load() event and resize() event are combined
  $(window)
  .ready(function() {
    responsiveWidth('d3-line', {top: 20, right: 20, bottom: 20, left: 40});
  })
  .resize(function() {
    responsiveWidth('d3-line', {top: 20, right: 20, bottom: 20, left: 40});
  });

requirements:
  - This function uses an associative javascript array function
    for the margin array. This can be found also in my gists.
  - You will also need jQuery for this.
  
*/

function responsiveWidth(objID, margin) {
  
  // set the responsive width of the screen
  factor = 0.35; // this example means 35% of 100% screen
  
  // get the browser dimensions
  width = $( window ).width();
  height = $( window ).height();
  
  // use a margin, if needed
  if (margin != undefined) {
    if ($.assocArraySize(margin) != 4) {
      var margin = {top: 0, right: 0, bottom: 0, left: 0};
    }
  } else {
    var margin = {top: 0, right: 0, bottom: 0, left: 0};
  }

  // calculate the new width in pixels 
  newWidth = Math.floor(width*factor)+margin['left']+margin['right'];

  // set the width of the given element by the html id
  document.getElementById(objID).setAttribute('width',newWidth);

  // return the width in addition to the change before so we can use the function also to initiate the width
  return newWidth;
}