code: function sn_ui_pixelWidth(selector) {
  	if ( selector === undefined ) {
  		return undefined;
  	}
  	var styleWidth = d3.select(selector).style('width');
  	if ( !styleWidth ) {
  		return null;
  	}
  	var pixels = styleWidth.match(/([0-9.]+)px/);
  	if ( pixels === null ) {
  		return null;
  	}
  	var result = Math.floor(pixels[1]);
  	if ( isNaN(result) ) {
  		return null;
  	}
  	return result;
  }
docs: Get the width of an element based on a selector, in pixels.
  
  @param {string} selector - a selector to an element to get the width of
  @returns {number} the width, or {@code undefined} if {@code selector} is undefined,
  or {@code null} if the width cannot be computed in pixels