shinyServer(function(input, output) {
  output$distPlot <- renderPlot({
 
    # mosaic setup
    require(mosaic); require(mosaicData)
    trellis.par.set(theme=theme.mosaic())

    # create new variable
    SAT = mutate(SAT, fracgrp = cut(frac, 
      breaks=c(0, 22, 49, 81), 
      labels=c("low fraction", "medium fraction", "high fraction")))

    # generate the desired plot
    if (input$stratify == "No") {
      xyplot(sat ~ salary, type=c("p", "r"), data=SAT)
    } else {
      xyplot(sat ~ salary, groups=fracgrp, auto.key=TRUE, 
        type=c("p", "r"), data=SAT)
    }

    })
  }
)