how to get position of where I clicked

[sfullman]

see attached picture. the orange "spinner" icon at the far left is designed to let user control to move the row up or down. It is 19px high. I want to write something like this:

<script language="JavaScript">
function indexRow(o,e){
   alert('position is '+ (o.offsetY))
}
</script>
<img id="index_1" src="images/buttons/spinner.png" onclick="indexRow(this,event);" height="19" />

Except, I don't know javascript well enough and obviously there is no such thing as "offsetY"

Can anyone help with this? thanks!

[sfullman]

I forgot the attachment - should help