I think you have hit the difference between a computer program and a linguistic study choice and actual sign.

Citation forms can be chosen to avoid the diagonal planes, but if you are transcribing from actual people signing, their hands and motions will be all over the place. If they are in conversation and are telling a story about someone up in a tree, writing precisely will show them moving their hands in diagonals from you on the ground to your brother in the tree. Throwing a ball in front of you, your hands end diagonal to the plane.  You can show it from the side to avoid the diagonal, but that becomes writing style, not real life. 

I think for a program you can choose not to show the third plane, but just know that you have done so. Technically, the sign for elephant or any other sign moving up and over or down and under is using the diagonal plane.

From: Ingvild Roald <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, November 5, 2010 7:48:31 AM
Subject: Re: how important is the diagonal plane of SW?

Here are 3 ways of writing FLY (aeroplane. flying by aeroplane) in Norwegian SL. The first one where only the wall plane is used, is good enough for ordinary use. The other two are for more special purposes, when you have to tell in which direction a plane is flying


(you may also look at the statics in 'serch by symbol' in the Norwegian Sign Puddle - the diagonal is hardly used, as you will see)

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