cascading of multiple plots

I notice that when generating several plots, they are

    > cascaded upon one another, one slightly to the right and
    > below of the previous one. However, when the plots get
    > near the bottom of the screen, they keep cascading even
    > when only the top of the plot can be seen. Should the
    > cascading not reset when the *bottom* of a frame touches
    > the bottom of the screen, and not the top?

    > Also, when the previous plot disappears prior to the
    > current plot, is there any need to cascade them at all?
    > Currently, the plots cascade even when prior plots have
    > disappeared.

Me thinks this is backend dependent behavior, since we don't do
anything explicit here we fall back on the default behavior for each
backend. Backend dependent patches welcome!