I don't think the demand should be shown between airport pairs, but within city pairs...
Players can ask their marketing deparment to estimate the demand between "London and Paris" and that should be shown in the charts.
Then how that demands splits between different airports should be the players job to figure out.
If your same priced flight out of Stansted gets half LFs than your flight out of Heathrow, then players should figure that out, just as they figure out why 2am flights are empty while 9am are full...
Sadly, that won't happen if the system is not dinamic. RL demand don't switch from airports "gradually", they just go to "edreams.com" or "priceline.com" and book the best fare in relation to "price, airport, time, etc, etc".
Proposal: everyday the system has a city pair demand (not airport, but city). Then it searches between all possible direct flights + connections, and assign demand according to how the system scored the flights (more points to direct 9am flights out of close to the city airports, less pointg to connecting flights out of far airports; more points to cheap flights, less to expensive, etc)