How to get coordinates from a R plotly figure

I have been struggling like mad to solve an apparently basic question. Imagine you have a scatter plot, with say ... 10 markers. I suppose this plot has been generated using plotly within a Shiny environment.

One can easily get the coordinates of these markers using the event_data("plotly_click") code.

Now imagine you do not need the coordinates of these markers, but the coordinates generated by a mouse click but precisely where no marker exists (for example because you would like to set a new marker exactly there, and you would like to re-use the information coming from that mouse click).

I cannot obtain such a behavior using onclick(), or whatever.

Any idea ?

Solution

You could add a D3 event listener to your plot

Plotly.d3.select('.plotly').on('click', function(d, i) {})

and then

retrieve the relative x and y values based on the click position (d3.event.layerX resp. layerY)
adjusting for the relative graph position (document.getElementsByClassName('bg')[0].attributes['x'])
and finally calculating the new values based on the axis ranges (myPlot.layout.xaxis.range[0])

The new x and y value are then pushed to the existing graph

Plotly.extendTraces(myPlot, {
      x: [[x]],
      y: [[y]]
    }, [1]);

Complete R code

library("plotly")
library("htmlwidgets")

p <- plot_ly(x = c( -2, 0, 2 ),y = c( -2, 1, 2), type = 'scatter' ,mode = 'lines+markers') %>% 
  add_trace(x=c(-1,0.4,2),y=c(2, 0, -1),type='scatter',mode='lines+markers') %>% 
  layout(hovermode='closest')
javascript <- "
var myPlot = document.getElementsByClassName('plotly')[0];
Number.prototype.between = function (min, max) {
  return this >= min && this <= max;
};

Plotly.d3.select('.plotly').on('click', function(d, i) {
  var e = Plotly.d3.event;
  var bg = document.getElementsByClassName('bg')[0];
  var x = ((e.layerX - bg.attributes['x'].value + 4) / (bg.attributes['width'].value)) * (myPlot.layout.xaxis.range[1] - myPlot.layout.xaxis.range[0]) + myPlot.layout.xaxis.range[0];
    var y =((e.layerY - bg.attributes['y'].value + 4) / (bg.attributes['height'].value)) * (myPlot.layout.yaxis.range[0] - myPlot.layout.yaxis.range[1]) + myPlot.layout.yaxis.range[1]
  if (x.between(myPlot.layout.xaxis.range[0], myPlot.layout.xaxis.range[1]) && 
y.between(myPlot.layout.yaxis.range[0], myPlot.layout.yaxis.range[1])) {
    Plotly.extendTraces(myPlot, {
      x: [[x]],
      y: [[y]]
    }, [1]);
  }
});"
p <- htmlwidgets::prependContent(p, onStaticRenderComplete(javascript), data=list(''))
p

Interactive Javascript example

var traces = [{
  x: [1, 2, 3, 4],
  y: [10, 15, 13, 17],
  mode: 'markers',
  type: 'scatter'
}];

traces.push({
  x: [2, 3, 4, 5],
  y: [16, 5, 11, 9],
  mode: 'markers',
  type: 'scatter'
});

traces.push({
  x: [1, 2, 3, 4],
  y: [12, 9, 15, 12],
  mode: 'markers',
  type: 'scatter'
});

traces.push({
  x: [],
  y: [],
  mode: 'lines+markers',
  type: 'scatter'
});

var myPlot = document.getElementById('myPlot')
Plotly.newPlot('myPlot', traces, {hovermode: 'closest'});

Number.prototype.between = function(min, max) {
  return this >= min && this <= max;
};

Plotly.d3.select(".plotly").on('click', function(d, i) {
  var e = Plotly.d3.event;
  var bg = document.getElementsByClassName('bg')[0];
  var x = ((e.layerX - bg.attributes['x'].value + 4) / (bg.attributes['width'].value)) * (myPlot.layout.xaxis.range[1] - myPlot.layout.xaxis.range[0]) + myPlot.layout.xaxis.range[0];
  var y = ((e.layerY - bg.attributes['y'].value + 4) / (bg.attributes['height'].value)) * (myPlot.layout.yaxis.range[0] - myPlot.layout.yaxis.range[1]) + myPlot.layout.yaxis.range[1]
  if (x.between(myPlot.layout.xaxis.range[0], myPlot.layout.xaxis.range[1]) &&
    y.between(myPlot.layout.yaxis.range[0], myPlot.layout.yaxis.range[1])) {
    Plotly.extendTraces(myPlot, {
      x: [
        [x]
      ],
      y: [
        [y]
      ]
    }, [3]);
  }
});

<script src="https://cdn.plot.ly/plotly-latest.min.js"></script>
<div id="myPlot" style="width:100%;height:100%"></div>

Shiny example

library(shiny)
library("plotly")
library("htmlwidgets")

ui <- fluidPage(
  plotlyOutput("plot")
)

server <- function(input, output) {
   javascript <- "
function(el, x){
  Number.prototype.between = function (min, max) {
   return this >= min && this <= max;
  };

  Plotly.d3.select('.plotly').on('click', function(d, i) {
  var e = Plotly.d3.event;
  var bg = document.getElementsByClassName('bg')[0];
  var x = ((e.layerX - bg.attributes['x'].value + 4) / (bg.attributes['width'].value)) * (el.layout.xaxis.range[1] - el.layout.xaxis.range[0]) + el.layout.xaxis.range[0];
  var y =((e.layerY - bg.attributes['y'].value + 4) / (bg.attributes['height'].value)) * (el.layout.yaxis.range[0] - el.layout.yaxis.range[1]) + el.layout.yaxis.range[1]
  if (x.between(el.layout.xaxis.range[0], el.layout.xaxis.range[1]) && y.between(el.layout.yaxis.range[0], el.layout.yaxis.range[1])) {
    Plotly.extendTraces(el, {
     x: [[x]],
     y: [[y]]
    }, [1]);
  }
});
}"
  output$plot <- renderPlotly({
    plot_ly(x = c( -2, 0, 2 ),y = c( -2, 1, 2), type = 'scatter' ,mode = 'lines+markers') %>% 
    add_trace(x=c(-1,0.4,2),y=c(2, 0, -1),type='scatter',mode='lines+markers') %>% 
    layout(hovermode='closest') %>% onRender(javascript)
  })
}

shinyApp(ui = ui, server = server)