Offer arrival date vs departure date view

[wcmille]

I think it would be useful to toggle the view so that you could see a plot of arrival date versus departure date instead of just travel time versus departure date.

When I send Jeb out there, sometimes I want to know what's the cheapest to get him there the soonest, and the current plot can sometimes be hard to figure that out.

Plot would be like these NASA plots: https://en.wikipedia.org/wiki/Porkchop_plot#/media/File:Porkchop_plot.gif