Two methods replacing each other (window.onload/Canvas JS)?

I haven't been working with javascript before, so I am still learning the basics.

I have included two javascript files in my main file called index.html. The reason for this is to let users select which javascript file to show (diagram 1 or diagram 2) by clicking on the navigation tool bar. The problem is that both of these javascript files have a equal method called window.onload, so they replace each other. How can I prevent these two javasript files to replace each other?

First javascript file:

window.onload = function {// Preview diagram 1}

Second javascript file:

window.onload = function {// Preview diagram 2}

Here is a snip of what I have in my index.html

<script type="text/javascript" src="js/diagram1.js"></script>
<script type="text/javascript" src="js/diagram2.js"></script>