How do I make a video animation with transparent background?

[Martin Schöön]

Here is what I want to do: I want to create an animated graph and
store it as a video file, mp4 say. This file will then be used as
an overlay on another video.

Using this example:
http://matplotlib.org/examples/animation/moviewriter.html
and this tutorial:
https://www.youtube.com/watch?v=h0F5X4b7jug
I have covered a lot of ground.

What I have failed to achieve is a graph with a transparent
background. None of the leads I have found searching the Internet
have helped me. (I may have found the needed information without
realising it...)

Any help is appreciated.

Another thing I would like to do is to present numerical data
in the video or as a separate video -- think LCD display reading
out numbers up-dated every second. Once again preferably on a
transparent background. Here I have covered no ground :-(

TIA,

/Martin