The Flatten to Plane plugin was the one with which I had the best results, although some of the lines are still not flattened (even when I select everything before activating the plugin).
The Drape tool didn't do anything for me.
I'm not sure, but I think that my problem is that not only I have lines in a diferent "Z-level" (lets say, still in the same plane as the plane made with the red and green axys, but some meters above 0), but some lines are in completely diferent planes... Those lines seems not to be affected by this plugin.
Still looking for a complete solution.
Thanks for the help so far.
Jose