It would be a graph with distance on x axis, and the elevation curves in different colors against the y axis.
And from a UI perspective I can imagine that in the tracks and the items tabs the user can select the tracks to be presented.
So Locus is not responsible if the selction makes sense or not. Keeping it simple for you. And flexible for the user.
The distance scale is defined by the longest track in question.
Needs one more feature though: we cannot expect that any two tracks are so much in synch that a point to point comparison based on distance to start makes any sense.
So a means is needed to define a point on each track and "associate" all of them as "identical" on x axis.
Example (an anomaly nobody would do, but it illustrates perfectly the tooling needed):
Track 1 is 20 km long, track 2 is cut out of track 1, startig at km 5, to km 15.
You select both for combined viewing.
If you "associate" the 5-km-point of track with with the 10-km-point of track one, track 2 shifts horizontally and you will see only one line (with color mix for the length of track 2).
Any real life situation can be handled with above mechanics.
Makes sense ?