Shiny savings app

server.R

library(shiny)
library(rHighcharts)

shinyServer(function(input, output) {
    
    .data <- reactive({
        interest <- 1 + ( as.integer(input$interest) / 100 )
        start <- as.numeric(input$start)
        monthly <- as.numeric(input$monthly)
        years <- as.integer(input$years)
        
        x <- start
        for (y in 1:years) {
            for (m in 1:12) { 
                start <- start + monthly * interest^(m/12) 
            }
            start <- start * interest
            x <- c(x, start)
        }
        
        return(x)
        
    })
    
    output$results <- renderChart({
        a <- rHighcharts:::Chart$new()
        a$chart(type = "column")
        a$legend(enabled = FALSE)
        a$title(text = "Forecast")
        a$yAxis(title = list(text = "Amount"))
        a$xAxis(title = list(text = "Years"))
        a$data(x = .data(), name = "Amount")
        return(a)
    })
    
})

ui.R

library(shiny)
library(rHighcharts)

shinyUI(pageWithSidebar(
    
    headerPanel(""),
    
    sidebarPanel(
        textInput("start", "Start amount", value = 0),
        textInput("monthly", "Monthly savings", value = 1000),
        sliderInput("interest", "Yearly interest rate (%)", min = 0, max = 20, value = 5, step = 1),
        sliderInput("years", "Number of years to show", min = 1, max = 100, value = 20, step = 1)
    ),
    
    mainPanel(
        chartOutput("results")
    )
))