Code for a reactive table to plot in Shiny

I'm new to shiny, and I am trying to do a simple density plot where there are 2 groups of data, with reactive 'shifts' in the mean etc. A simplified summary of this is that one set of data has a mean of 0, and variance of 1. The second set of data has a mean of shift, which is defined in a slider.

I have tried to use reactiveValues, as shown in the code below to store the matrix of observations d1, generated from the density function y values, and the corresponding x values are stored in x.

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

   # Application title
   titlePanel("Old Faithful Geyser Data"),

   # Sidebar with a slider input for number of bins 
   sidebarLayout(
      sidebarPanel(
         sliderInput("shift",
                     "shift of 2nd set",
                     min = -1,
                     max = 1,
                     value = 0)
      ),

      # Show a plot of the generated distribution
      mainPanel(
         plotOutput("distPlot")
      )
   )
)

# Define server logic required to draw a histogram
server <- function(input, output) {
   data <- reactiveValues({

     d1 <- matrix(nrow=100, ncol=512)
     for(i in 1:70){
       d1[i,] <- density(rnorm(1000),from = -3, to = 3)$y
     }
     for(i in 71:100){
       d1[i,] <- density(rnorm(1000, input$shift),from = -3, to = 3)$y
     }
     x <- density(rnorm(1000),from = -3, to = 3)$x

   })
   output$distPlot <- renderPlot({
      matplot(data$x, t(data$d1), type = "l", lty = 1, col = c(rep(1,70),rep(2,30)))
   })
}

# Run the application 
shinyApp(ui = ui, server = server)

The above code is largely from the example shiny app, so please excuse any generic references. It should still work.

I was expecting a shiny plot with a slider on the left, and a plot on the right with 100 density lines in 2 colours. When the shift slider is changed the second set of data (red) will slide left or right depending on the shift.

Instead, I get the error message

  55: stop
  54: .getReactiveEnvironment()$currentContext
  53: .subset2(x, "impl")$get
  52: $.reactivevalues
  47: server [/beavis/Documents/test/app.R#37]
Error in .getReactiveEnvironment()$currentContext() : 
  Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.)

Is anyone able to help me fix this code? Any help will be greatly appreciated. Having played around for an hour I believe the issue lies in the reactiveValues section, but nothing so far has worked.