Vertical dashed line conecting notes

[Caio Barros]

Hi,
I'm writing a composition with two unmeasured staffs and I want to draw a vertical dashed line that indicates when the notes sound together.
I've tried something like that:

\version "2.12.1"
\relative c' {
 <<
  { c_\markup { \draw-line #'(0 . 9)  } }
  { a }
 >>
}

but as you can see there are some problems:
- the line doesn't draw itself at the middle of the notehead
- the botton staff moves itself lower so the line does not colide. (I want it do colide!)
- I don't know how to draw a dashed line with the \draw-line command ;)

Is there a way to tweak this, or maybe is better to use another code?

Thank you
Caio Barros