Allow As Of selection by clicking on evaluation plot

It would be convenient to allow the user to select an As Of date by having them click on a particular point in an evaluation plot (any of 1-4 weeks ahead). The point they click on would correspond to a target date that we then translate into an As Of selection. (I'm trying to use the As Of feature now myself to understand the behavior of two forecasters and I find myself doing lots of hovering, squinting, and scrolling to get what I want.)

Start by locating the evaluation plot entry point and the existing As Of selection behavior in the repository. Trace how plotted points represent target dates and how a selected date is translated into an As Of value; done means clicking a point selects the corresponding As Of date for plots one to four weeks ahead.

Written by the indexing model from the issue text.