Scratch

Bah humbug. I was afraid that somebody might ask. It is possible, but as ErnieParke pointed out, you won't be able to preview it as you can create 1 preview but then cannot clear it as this will destroy the rest.

Usually art is single-rendered, meaning that the pen marks are made and left alone. The only way you can also have this feature is dynamic-rendered (a.k.a multi-rendered or polyrender). This is when pen mark data is stored in a list, and constantly redrawn in single-frame. This allows for single objects to be selected, removed, or modified; it's the SVG of the Scratch world. One script renders the list repeatedly, while another modifies it's contents. With this method, updating previews and other features like that are easy. If you would like an example, just ask. 

Here:

pen up
set [ox v] to (mouse x)
set [oy v] to (mouse y)
wait until <not <mouse down?>>
set [nx v] to ((mouse x) - (ox))
set [ny v] to ((mouse y) - (ox))
repeat until <(nx) < (0.1)>
 pen up
 set [step v] to [0]
 go to x: (ox) y: (oy)
 pen down
 repeat (4)
  go to x: (([sin v] of (step)) * (nx)) y: (([cos v] of (step)) * (ny))
  change [step v] by (1)
  ^^ repeat the above 2 blocks 90 times ^^
 end
 change [ox v] by (-0.1)
 change [oy v] by (-0.1)
end

berberberber wrote:

How does it work?

It draws an ellipse.

nx is the x-axis of the ellipse, ny is the y-axis of the ellipse. (ox, oy) is the center.

ill make you a circle tool