Understanding the Issue with plotly_selected in R Shiny with Plotly

When developing interactive applications using R Shiny and Plotly, it’s essential to understand how to capture user interactions such as selecting points on a scatter plot. In this article, we’ll delve into the issue of not capturing the plotly_selected event and provide solutions to achieve the desired behavior.

Background: Event Registration in Plotly

Before diving into the solution, let’s briefly discuss event registration in Plotly. When using Plotly with R Shiny, events such as hoverover, click, and selection can be registered using the event_register() function. This function allows us to capture specific events and update our application accordingly.

In the given code snippet, we’re trying to use the plotly_selected event to capture selections on a scatter plot. However, the warning message indicates that this event is not registered for the Plotly source ID “plotPCA”.

Event Registration: The Missing Piece

To fix the issue, we need to register the plotly_selected event for the “plotPCA” source ID using the event_register() function.

event_register(p, 'plotly_selected')

We can add this line of code after creating the Plotly output and before defining the observeEvent handler that captures the selection events.

Modifying the Observe Event Handler

The given code snippet already attempts to capture selections by using observeEvent(). However, there’s a crucial difference between capturing specific event types (e.g., hoverover) and general selection events.

To fix this issue, we need to modify the observeEvent handler to specifically capture the ‘plotly_selected’ event.

observeEvent(
  eventExpr = "event_data('plotly_selected', source='plotPCA')", 
  handlerExpr = {
    # code to handle selected points goes here
  }
)

By making this change, we can ensure that our observeEvent handler captures the ‘plotly_selected’ event for the Plotly source ID “plotPCA”.

Creating a Function to Update Reactive Values

In addition to registering the event and modifying the observeEvent handler, we need to create a function that updates the reactive values based on user selections.

Here’s an updated code snippet that demonstrates how to achieve this:

# Create a function to update reactive values
update_reactive_values <- function(selected_points) {
  # Get the selected points
  selected_point_numbers <- event_data("plotly_selected", source = "plotPCA")$pointNumber
  
  # Update the dat.res reactive value
  if (!is.null(selected_point_numbers)) {
    dat.res <- data[selected_point_numbers,]
  } else {
    dat.res <- isolate(getPCAdata())
  }
  
  # Return the updated reactive values
  return(list(dat.res = dat.res))
}

# Register the plotly_selected event
event_register(p, 'plotly_selected')

# Define the observeEvent handler
observeEvent(
  eventExpr = "event_data('plotly_selected', source='plotPCA')", 
  handlerExpr = {
    # Call the function to update reactive values
    selected_points <- event_data("plotly_selected", source = "plotPCA")$pointNumber
    dat.res <- update_reactive_values(selected_points)
    getPCAdata <- dat.res$dat.res
  }
)

# Define the plot output
output$plotPCA <- renderPlotly({
  # Create a reactive value to store PCA data
  dat.tmp <- getPCAdata
  
  # Check if PCA data is available
  if (!is.null(dat.tmp)) {
    # Get the color group based on user selection
    color_group = ~get(input$selectColorForPCA)
    
    # Plot the scatter plot
    plot_ly(
      dat.tmp,
      x = ~PC1,
      y = ~PC2,
      color = color_group,
      type = 'scatter',
      mode = 'markers',
      hoverinfo = 'text',
      text = ~SampleID,
      source='plotPCA'
    )
  }
})

Conclusion

In this article, we’ve explored the issue of not capturing the plotly_selected event in R Shiny with Plotly. We’ve discussed the importance of registering events and modifying observeEvent handlers to capture specific user interactions.

By following the steps outlined above, you can create interactive scatter plots that update reactive values based on user selections. Remember to register events and modify your observeEvent handler accordingly to achieve the desired behavior in your R Shiny applications with Plotly.

Last modified on 2025-01-26