This is a replacement for the shopify size-chart snippet that allows a different size chart to be used per vendor and requires a page called vendor-name-size-chart to work

shopify-brand-size-chart

{% if template contains 'product' %}
{% assign chart = product.vendor | handleize | append: "-size-chart" %}
{% unless pages[chart].empty? or pages[chart].content == blank %}
{% assign size_options = 'size,taille' | split: ',' %}
{% assign size_chart_text_link = 'Size chart' %}
{% assign has_size = false %}
{% assign size_index = 0 %}
{% for option in product.options %}  
  {% assign downcased_option = option | downcase %}
  {% if has_size == false and size_options contains downcased_option %}
    {% assign has_size = true %}
    {% assign size_index = forloop.index0 %}
  {% endif %}
{% endfor %}
{% if has_size %}
<script>
/*
* $ lightbox_me
* By: Buck Wilson
* Version : 2.3
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
(function(a){a.fn.lightbox_me=function(b){return this.each(function(){var e=a.extend({},a.fn.lightbox_me.defaults,b),l=a(),k=a(this),m=a('<iframe id="foo" style="z-index: '+(e.zIndex+1)+';border: none; margin: 0; padding: 0; position: absolute; width: 100%; height: 100%; top: 0; left: 0; filter: mask();"/>'),g=/MSIE 6/.test(navigator.userAgent)&&!window.opera;if(e.showOverlay){var f=a(".js_lb_overlay:visible");if(f.length>0){l=a('<div class="lb_overlay_clear js_lb_overlay"/>')}else{l=a('<div class="'+e.classPrefix+'_overlay js_lb_overlay"/>')}}if(g){var d=/^https/i.test(window.location.href||"")?"javascript:false":"about:blank";m.attr("src",d);a("body").append(m)}a("body").append(k.hide()).append(l);if(e.showOverlay){h();l.css({position:"absolute",width:"100%",top:0,left:0,right:0,bottom:0,zIndex:(e.zIndex+2),display:"none"});if(!l.hasClass("lb_overlay_clear")){l.css(e.overlayCSS)}}if(e.showOverlay){l.fadeIn(e.overlaySpeed,function(){c();k[e.appearEffect](e.lightboxSpeed,function(){h();c();e.onLoad()})})}else{c();k[e.appearEffect](e.lightboxSpeed,function(){e.onLoad()})}if(e.parentLightbox){e.parentLightbox.fadeOut(200)}a(window).resize(h).resize(c).scroll(c).keyup(j);if(e.closeClick){l.click(function(n){i();n.preventDefault})}k.delegate(e.closeSelector,"click",function(n){i();n.preventDefault()});k.bind("close",i);k.bind("reposition",c);function i(){var n=k[0].style;if(e.destroyOnClose){k.add(l).remove()}else{k.add(l).hide()}if(e.parentLightbox){e.parentLightbox.fadeIn(200)}m.remove();k.undelegate(e.closeSelector,"click");a(window).unbind("reposition",h);a(window).unbind("reposition",c);a(window).unbind("scroll",c);a(document).unbind("keyup",j);if(g){n.removeExpression("top")}e.onClose()}function j(n){if((n.keyCode==27||(n.DOM_VK_ESCAPE==27&&n.which==0))&&e.closeEsc){i()}}function h(){if(a(window).height()<a(document).height()){l.css({height:a(document).height()+"px"});m.css({height:a(document).height()+"px"})}else{l.css({height:"100%"});if(g){a("html,body").css("height","100%");m.css("height","100%")}}}function c(){var o=k[0].style;k.css({left:"50%",marginLeft:(k.outerWidth()/2)*-1,zIndex:(e.zIndex+3)});if((k.height()+80>=a(window).height())&&(k.css("position")!="absolute"||g)){var n=a(document).scrollTop()+40;k.css({position:"absolute",top:n+"px",marginTop:0});if(g){o.removeExpression("top")}}else{if(k.height()+80<a(window).height()){if(g){o.position="absolute";if(e.centered){o.setExpression("top",'(document.documentElement.clientHeight || document.body.clientHeight) / 2 - (this.offsetHeight / 2) + (blah = document.documentElement.scrollTop ? document.documentElement.scrollTop : document.body.scrollTop) + "px"');o.marginTop=0}else{var p=(e.modalCSS&&e.modalCSS.top)?parseInt(e.modalCSS.top):0;o.setExpression("top","((blah = document.documentElement.scrollTop ? document.documentElement.scrollTop : document.body.scrollTop) + "+p+') + "px"')}}else{if(e.centered){k.css({position:"fixed",top:"50%",marginTop:(k.outerHeight()/2)*-1})}else{k.css({position:"fixed"}).css(e.modalCSS)}}}}}})};a.fn.lightbox_me.defaults={appearEffect:"fadeIn",appearEase:"",overlaySpeed:250,lightboxSpeed:300,closeSelector:".close",closeClick:true,closeEsc:true,destroyOnClose:false,showOverlay:true,parentLightbox:false,onLoad:function(){},onClose:function(){},classPrefix:"lb",zIndex:999,centered:false,modalCSS:{top:"40px"},overlayCSS:{background:"black",opacity:0.3}}})(jQuery);
Shopify.OptionSelectors.prototype.buildSelectors=function(){for(var c=0;c<this.product.optionNames().length;c++){var e=new Shopify.SingleOptionSelector(this,c,this.product.optionNames()[c],this.product.optionValues(c));e.element.disabled=false;this.selectors.push(e)}var b=this.selectorDivClass;var a=this.product.optionNames();var d=Shopify.map(this.selectors,function(f){var i=document.createElement("div");i.setAttribute("class",b);if(a.length>1){var h=document.createElement("label");h.htmlFor=f.element.id;h.innerHTML=f.name;i.appendChild(h)}i.appendChild(f.element);if(f.index==={{ size_index }}){var g=jQuery('<a id="size-chart-trigger" href="#size-chart">'+{{ size_chart_text_link | json }}+"</a>");g.click(function(){jQuery("#size-chart").lightbox_me({centered: true});return false});jQuery(f.element).after(g)}return i});return d};
</script>
<div id="size-chart" style="display:none" class="rte">{{ pages[chart].content }}</div>
<style>
#size-chart-trigger { padding-left: 8px }
#size-chart { background:white; padding:25px; text-align:center; z-index:10000 !important; }
</style>
{% endif %}
{% endunless %}
{% endif %}

Hi there Daniel,
It would be great if you can tell us how is this implemented.
Thanks