"Chamfered" Routing in Eagle
There is no way to make a trace with a gradual smooth linear change in width like that in Eagle.
(edit: Altium allows some control over the shape of the transition region between two traces, which in Altium is called teardrop.  Thank you @Mario, for this insight.)

It's possible to approximate this with a trace consisting of 2 segments with different widths.  But then the transition "ramps" are going to be circular, rather than linear.

It's possible, however, to create geometry exactly like that with a polygon copper pour.