Only certain plot points responding to row selection in shiny app? - r

Question

I am attempting to adapt an example (2.1.1) from the tutorial found here on interactive plots in shiny. I have a shiny app as follows:
Data:
seats = data.table(
ID = c("1","2","3","4","5","6"),
Row = c("A", "A", "A", "B", "B", "B"),
SeatNum = c(1,2,3,1,2,3),
y = c(1,1,1,2,2,2),
price = 45)
ui.R
fluidPage(
title = 'Select Table Rows',
h1('A Client-side Table'),
fluidRow(
column(6, DT::dataTableOutput('x1')),
column(6, plotOutput('x2', height = 500)),
column(3, verbatimTextOutput('x4')),
column(4, verbatimTextOutput('x5'))
),
hr(),
)
server.R
shinyServer(function(input, output, session) {
output$x1 = DT::renderDataTable(seats, editable = "row", server = FALSE)
# highlight selected rows in the scatterplot
output$x2 = renderPlot({
s = input$x1_rows_selected
par(mar = c(4, 4, 1, 1))
plot(seats$SeatNum, seats$y)
if (length(s)) points(seats[s, , drop = FALSE], pch = 19, cex = 2)
})
output$x4 = renderPrint({
s = input$x1_rows_selected
if (length(s)) {
cat('Combined price \n of all seats:\n\n')
cat(sum(seats[s,]$price))
}
})
output$x5 = renderPrint({
s2 = input$x1_rows_selected
if (length(s2)) {
cat('Total number of seats selected:\n\n')
cat(length(s2))
}
})
})
Upon launching the app I can select any of the first three rows and the plot reacts appropriately. However, from row 4 onwards the plot does not respond. I've played around with the if (length(s)) points(seats[s, , drop = FALSE], pch = 19, cex = 2) line but I don't understand its behavior.

Answer 1

The table used in the linked example has two columns only, so for the points function its unambiguous what values go to x and y. But your table has multiple columns, so the indexing seats[s, , drop=FALSE] returns the selected rows and all columns:
> seats[1:2, , drop=FALSE]
ID Row SeatNum y price
1: 1 A 1 1 45
2: 2 A 2 1 45
So, when indexing this way, points does not know what to map to x and y. You need to index the selected rows and the columns needed for points (in the correct order) to highlight points on the plot:
> seats[1:2, c("SeatNum", "y"), drop=FALSE]
SeatNum y
1: 1 1
2: 2 1
Working app:
library(shiny)
library(data.table)
library(DT)
seats = data.table(
ID = c("1", "2", "3", "4", "5", "6"),
Row = c("A", "A", "A", "B", "B", "B"),
SeatNum = c(1, 2, 3, 1, 2, 3),
y = c(1, 1, 1, 2, 2, 2),
price = 45
)
ui <-
fluidPage(title = 'Select Table Rows',
h1('A Client-side Table'),
fluidRow(
column(6, DT::dataTableOutput('x1')),
column(6, plotOutput('x2', height = 500)),
column(3, verbatimTextOutput('x4')),
column(4, verbatimTextOutput('x5'))
),
hr())
server <- function(input, output, session) {
output$x1 = DT::renderDataTable(seats, editable = "row", server = FALSE)
# highlight selected rows in the scatterplot
output$x2 = renderPlot({
s = input$x1_rows_selected
par(mar = c(4, 4, 1, 1))
plot(seats$SeatNum, seats$y)
if (length(s))
points(seats[s, c("SeatNum", "y") , drop = FALSE], pch = 19, cex = 2)
})
output$x4 = renderPrint({
s = input$x1_rows_selected
if (length(s)) {
cat('Combined price \n of all seats:\n\n')
cat(sum(seats[s, ]$price))
}
})
output$x5 = renderPrint({
s2 = input$x1_rows_selected
if (length(s2)) {
cat('Total number of seats selected:\n\n')
cat(length(s2))
}
})
}
shinyApp(ui, server)