Drawing function query

Function Dibujo(guibank)
	canvas=PeekInt(guibank,4)
	draw=PeekInt(guibank,12)
	
	Select EventID()
	
		Case $203
			If ButtonState(draw)
				SetBuffer CanvasBuffer(canvas)
					Plot EventX(),EventY()
				FlipCanvas canvas
			EndIf
			
	End Select
End Function 

As you see, I wiped half the function contents. Also, I replaced MouseX and MouseY with EventX and EventY. And I narrowed down the opened canvasbuffer to strictly the area where it's needed.

I wouldn't base a designtool on this structure tho. Correct way is to draw into an image buffer and draw this imagebuffer onto a canvas.
Additionally, you want to use the events $201 and $202 for mousedown and mouseup rather than a draw checkbox.