Seeking explanation on renderPlot in Shiny - r

Question

I have a trivial question after playing with this code
library(gapminder)
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
checkboxInput("fit", "Add line of best fit", FALSE),
selectInput("continents", "Continents",
choices = levels(gapminder$continent),
multiple = TRUE,
selected = 'Europe')
),
mainPanel(
plotOutput("plot")
)
)
)
server <- function(input, output) {
output$plot <- renderPlot({
data <- subset(gapminder,
continent %in% input$continents)
p <- ggplot(data, aes(gdpPercap, lifeExp)) +
geom_point() +
scale_x_log10()
if (input$fit) {
p <- p + geom_smooth(method = "lm")
}
p
})
}
shinyApp(ui = ui, server = server)
If I change part of the code above to this
if (input$fit) {
p + geom_smooth(method = "lm")
}
# p
})
}
The plot won't be displayed. Why do we have to be too verbose, that is, passing p + geom_smooth() to p, then calling p for it to work? Why can't we be more succinct by simply calling p + geom_smooth()? Is it because the code is wrapped inside renderPlot()?
Thankyou.

Answer 1

Based on your if statement, the plot is returned and then rendered in case input$fit is TRUE. If input$fit is FALSE, the if statement returns NULL and therefore no plot is rendered. This is the default behaviour of if() in R.
You can solve your problem by simply add else p to your if statement like
if (input$fit) p + geom_smooth(method = "lm") else p