I've been trying to do this for a few hours now.
Basically, I want to, when object A has a onmouseover event, It's the atributes of object B to change.
To depict it visually, if my pointer is over the gray square, I'd like the orange one to change color, or to have diferent opacity.

What's the easiest way to do this?

not tested

<rect id="grayone" onmouseover="document.getElementById('orangeone').style.fill='red'" ...>
<rect id="orangeone" ...>

Nnnop.
There's an extension with the portable release (0.92) that does this automatically, but it adds a metric ton of javascript AND only has worked in a years old version of chrome so far, 42.0. All other browsers I've tried have varying errors.

Try this file: