Allow As Of selection by clicking on evaluation plot

[ryantibs]

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.)

[nmdefries]

Resources for this feature:

• R Shiny Interactive Plots
• Change plot based on click input in R shiny