@jchenaud
MODULE_DOM_CREATED is not so good time. Because, It would be emitted when Your DOM was created, but All of other DOMs are not rendered yet. DOM_OBJECTS_CREATED is better.
If you want to using position, use .getDom(). A usual way to use position is, in .getDom() to initialize your rendering and to prepare the framework to draw.
And you can choose one of drawing tricks.

getDom() shall take care of all the drawing. in .getDom() put all of your rendering framework and contents. and you can refresh your render by calling .updateDom()

getDom() shall draw only framework, Contents should be drawn by your another function with HTML DOM manipulation.

or getDom() just return empty wrapper, Then, all of your rendering and refreshing will be performed by yourself.

Which is better? case-by-case.

For the last question; I cannot figure out. :)

For anything involving a graph drawn from JS(ON) data, I can only recommend highcharts.

It’s possible to display dynamic graphs from live data. Example: https://www.highcharts.com/stock/demo/dynamic-update/
https://www.highcharts.com/docs/working-with-data/live-data

Your python code could provide a JSON and the MM-module calls the JSON and prints the graph.

I’ve used highcharts before but never dynamically like in the example above.
Here’s an example where I used it in a module: https://github.com/TTigges/MMM-Oiltank/blob/master/MMM-Oiltank.js