Conditional reactivity Shiny - r

Question

Reactive expressions in Shiny propagate changes where they need to go. We can suppress some of this behaviour with isolate, but can we suppress changes being propagated based on our own logical expression?
The example I give is a simple scatterplot, and we draw a crosshair with abline where the user clicks. Unfortunately, Shiny considers the result to be a new plot, and our click value is reset to NULL... which in turn is treated as an update to the value to be propagated as usual. The plot is redrawn, and NULL is passed to both arguments of abline.
My hack (commented out below) is to place a condition in the renderPlot call which updates some non-reactive variables for the plotting coordinates, only when the click values are non-NULL. This works fine for trivial plots, but it actually results in the plot being drawn twice.
What's a better way to do this? Is there a correct way?
Server file:
library(shiny)
shinyServer(function (input, output)
{
xclick <- yclick <- NULL
output$plot <- renderPlot({
#if (!is.null(input$click$x)){
xclick <<- input$click$x
yclick <<- input$click$y
#}
plot(1, 1)
abline(v = xclick, h = yclick)
})
})
UI file:
library(shiny)
shinyUI(
basicPage(
plotOutput("plot", click = "click", height = "400px", width = "400px")
)
)

Answer 1

Winston calls this problem "state" accumulation - you want to display not only the current data, but something generated by the previous plot (the best place to learn about this is at https://www.rstudio.com/resources/videos/coordinated-multiple-views-linked-brushing/)
The basic idea is to create your own set of reactive values, and update them when the user clicks on the plot. They won't be invalidated until the next click, so you don't get circular behaviour.
library(shiny)
shinyApp(
shinyUI(basicPage(plotOutput("plot", click = "click"))),
function(input, output) {
click <- reactiveValues(x = NULL, y = NULL)
observeEvent(input$click, {
click$x <- input$click$x
click$y <- input$click$y
})
output$plot <- renderPlot({
plot(1, 1)
abline(v = click$x, h = click$y)
})
}
)