Using existing cairo context for GtkDrawingArea

[asubedi <asubedi gmail com>]

Hi,

I am trying to create a Graph widget using Cairo. I am using Ruby, but
you can reply in any language you prefer.

In the examples I have seen about using cairo in gtk application, most
create a cairo context from GtkDrawingArea in the handler of expose
event. However, I would like to create a cairo context independent of
GtkDrawingArea, and would like to use that context in GtkDrawingArea's
expose handler.  How can I do that?