Interactive Graphics

Lecture 25

Dr. Eric Friedlander

College of Idaho
CSCI 2025 - Winter 2026

Interactive Plots

The Concept

• Static Plots: Display data (output).
• Interactive Plots: Display data AND accept user input (output + input).
• Mechanism:
  • plotOutput() can accept pointer events (clicks, double clicks, hovers, brushes).
  • Sends these events to the server as input$ID.

Clicking

• UI: Add click = "ID" to plotOutput().
• Server: Access input$ID (contains x, y coordinates).
• Helper: nearPoints() returns rows from the dataframe near the click.

ui <- fluidPage(
  plotOutput("plot", click = "plot_click"),
  tableOutput("data")
)

server <- function(input, output, session) {
  output$plot <- renderPlot({
    ggplot(mtcars, aes(wt, mpg)) + geom_point()
  }, res = 96)
  
  output$data <- renderTable({
    nearPoints(mtcars, input$plot_click)
  })
}

Brushing

• Brush: A rectangular selection tool.
• UI: Add brush = "ID" to plotOutput().
• Helper: brushedPoints() returns rows inside the brush.

ui <- fluidPage(
  plotOutput("plot", brush = "plot_brush"),
  tableOutput("data")
)

server <- function(input, output, session) {
  output$data <- renderTable({
    brushedPoints(mtcars, input$plot_brush)
  })
}

Plot Options

• click: Single click.
• dblclick: Double click.
• hover: Mouse hover (triggers frequently).
• brush: Drag-to-select rectangle.
  • Can constrain to x or y axis: brush = brushOpts(direction = "x").

Linked Plots

• Goal: interacting with one plot changes another.
• Pattern:
  1. Create a plot with a brush.
  2. Use brushedPoints() to filter the data.
  3. Create a second plot (or table) based on the filtered data.
• Result: Highlighting or zooming into data subsets.

Wrap Up

Mechanics

• UI: plotOutput(..., click = "click_id", brush = "brush_id").
• Server Helpers:
  • nearPoints(df, input$click_id): Find single point.
  • brushedPoints(df, input$brush_id): Find multiple points.
• Power: Enables “drill-down” data exploration.

Do Next

1. Read Chapter 7: Graphics from Mastering Shiny.
2. There’s NO recitation Gem for this textbook but I recommend creating your own and adding the textbook chapter and these slides.
3. I reommend working through the Exercises in the textbook.
4. Move on to Lecture 25!