plotOutput now takes clickId and hoverId params that tell Shiny
where to send click and hover events for that plot. The server.R
file can listen on input$<clickId> and/or input$<hoverId>. In
both cases, the resulting value will have numeric x and y elements
that indicate the mouse position in user coordinates. In the case
of hover events, it's also possible to have a NULL value which
means the mouse is not currently hovering over the plot.