Graph cdf of probability of selecting among the biggest k exits

Now that we have our scripts in #6232 (moved) extracting useful data, here's another graph that would be useful.

On the x axis is our set of 900 Exit relays, ordered by chance of being chosen. f(x) is the chance that the user's selected exit is in the first (biggest) x relays.

We'll likely find that we should zoom in on just the x \in [0..50] range or something, since otherwise the graph will just shoot up to 1.0 and stay there.

Auto generating this graph for the current consensus, and sticking it near consensus-health, might be wise.

Once we've done the basic graph, we might find that graphing f(10) over time tells us something interesting about #6232 (moved) (for various values of 10).