You would still have the regular graphs and see that the average is much higher or lower than the one way route and to check the return leg. The other way to do it would be just to display the difference with larger numbers being more asymmetrical. Either way I think showing the outbound and return legs on the same page will create confusion.
Maybe you are misunderstanding the request. It is not about charts. Just 4 labels, 4 text values:
Potential Outgoing: Value 1
Actual Outgoing: Value 2
Potential Returning: Value 3
Actual Returning: Value 4
It could also be 2x2 table.
These values would in upper portion of the page where we now have a single value Estimated Cargo Demand
The chart at the bottom of the page would stay as is, showing only the outgoing demand.
There could possibly be some Super Duper implementation down the road, but why go another 2 years with insufficient data and a clickfest, when a simple solution, probably taking 2 hours of programming can serve us 2 years, before Sami can potentially get to some alternate presentation.