[cairo] drawing on top of another drawing

[Diego Jacobi]

Hello.
I am starting to learn cairo. Currently i am working in a proyect
"Fase Analyzer" where i draw some stuff (wave form, spectrum, etc.) in
a gtk drawing area.

I now need to show some information about each waveform when mouse is
over each one.
I think in drawing a tiny box (like a Hint) showing the wave values
for the point where mouse resides.

How do you solve that with cairo?

I can think in 2 methods but i dont know how to go for them with cairo yet.

a) using layers and drawing the tiny box in a second, almost always
empty, layer.
b) save the content of a region and draw over it the tiny box, and
when mouse moves away, recover the buffer.

Cheers.
Diego