I figured out how to catch clicks on my champlain map. (left mouse click is still dodgy as the button release only is triggered when you move the mouse after releasing).
It turned out I had to call clutter_actor_set_reactive() on the map. Not knowing clutter at all, it would have been nice if the champlain docs mentioned this.
But now in gpx-viewer-BZR you can click on the track, to see that point highlighted in the graph. The next thing I want to figure out is how to easily see a plot of 2 combined tracks. I am thinking of only showing tracks that are selected, and allow you to select multiple tracks. But no idea how other people use gpx-viewer.