For https://community.rstudio.com/t/updating-a-plotly-chart-in-shiny-using-flexdashboard/27176

leaflet-proxy-rmd.Rmd

---
title: "Untitled"
output: html_document
runtime: shiny_prerendered
---

```{r}
library(leaflet)
leafletOutput("map")
```


```{r, context="server"}
output$map <- renderLeaflet({
  addTiles(leaflet())
})
```


```{r, context="server"}
observe({
  invalidateLater(500)
  leafletProxy("map", session) %>%
    addCircles(
      rnorm(1, sd = 100), 
      rnorm(1, sd = 100), 
      layerId = "a"
    )
})
```