The AutoPageSetup object contains all the parameters in the plot window Page Setup dialog and is accessed from Document object's PageSetup property.
The following script demonstrates how the AutoPageSetup object is used in reference to the Document object.
'Declares GrapherApp as an object
Dim GrapherApp As Object
'Creates an instance of the Grapher Application
'object and assigns it to the variable named "GrapherApp"
Set GrapherApp = CreateObject("Grapher.Application")
'Makes Grapher visible
GrapherApp.Visible = True
'Declares Plot as an object
Dim Plot As Object
'Creates a plot document in Grapher and
'assigns it to the variable named "Plot"
Set Plot = GrapherApp.Documents.Add(grfPlotDoc)
'Declares PageSetup as an object
Dim PageSetup As Object
'Assigns the plot document page setup
'to the variable named "PageSetup"
Set PageSetup = Plot.PageSetup