redrawing plots dynamically in r - r

Question

I went a totally different direction in this project -- the issue I have is down at the end I need to clear out the graph when a different variable is selected. The graph as it is stays the same. Thanks.
I am not even sure how this would be phrased in the documents -- rewriting graphs, dynamic plotting ??? I saw display.removePlot(display.activePlotIndex()) but am not sure about that -- what do I look up to figure this out?
library(shiny)
library(DT)
library(ggplot2)
oboler_data <- read_csv(file = "C:/Users/12083/Desktop/ref.csv")
rdate <- as.Date(oboler_data$DATE,"%m/%d/%y")
ui <- fluidPage(sidebarLayout(
sidebarPanel(
selectInput("dataset", "choose a dataset", c("oboler_data")),
selectInput("column", "Type of Transaction", "placeholder1"),
selectInput("level", "select level", "placeholder2")
),
mainPanel(tableOutput("table"), plotOutput("Histo"))
))
server <- function(input, output, session){
dataset <- reactive({
get(input$dataset)
})
observe({
updateSelectInput(session, "column", choices = names(dataset()))
})
observeEvent(input$column, {
column_levels <- as.character(sort(unique(
dataset()[[input$column]]
)))
updateSelectInput(session, "level", choices = column_levels)
})
output$table <- renderTable({
subset(dataset(), dataset()[[input$column]] == input$level)
})
DF <- data.frame(Name = LETTERS[1:10], Value = rnorm(20), Value2 = runif(20))
output$TableOut <- renderDataTable({
DF
})
output$Histo <- renderPlot({
ggplot(DF, aes(Value)) + geom_histogram(binwidth = 0.1,
fill = "blue", color = "white")
})
}
shinyApp(ui, server)

Answer 1

I think you should use req, as it precludes (and clears!) rendering of a plot if conditions are not met.
library(shiny)
shinyApp(
ui = fluidPage(
checkboxInput("cb", "Plot?"),
sliderInput("cyls", "Cylinders", min = 1, max = 8, value = 4, step = 1),
plotOutput("plt")
),
server = function(input, output, session) {
output$plt <- renderPlot({
req(input$cb, input$cyls)
ggplot(mtcars[mtcars$cyl >= input$cyls,,drop = FALSE],
aes(disp, mpg, color = factor(cyl))) +
geom_point()
})
}
)
The three screenshots above are in sequence: start with "Plot?" deselected, no plot shown; select it, plot appears; deselect it, existing plot disappears.
The way you adapt this to you is to replace the req(input$cb) with something that matches your condition of "clear the plot". (I also included input$cyls here just to prevent larger more-complex apps from reaching the plot block before all inputs have stabilized. Perhaps not required in most apps, but it's a defensive move that carries little consequence if it is overkill, but lots of benefit when it is needed.)
A slight modification on a theme, "telling the user why the plot disappeared". Replace the req with a validate and at least one need:
# req(input$cb, input$cyls)
validate(
need(input$cb, "You deselected \"Plot!\"")
)