Shiny: Reactive sliderInput not working :(

Question

I'm making an R Shiny app where the graph type switches between line and stacked area. I want to have a slider which fliters the input data by y-axis values. When the graph type is stacked, the maximum y-axis value is obviously a lot higher than for a line graph. How do I update the maximum value for the slider when the graph type changes? As shown in the code below, I'm trying to use observeEvent, but this isn't having the desired effect. Any help would be greatly appreciated (I acknowledge that plotly could handle some of this but in general, I don't like the package).

library(shiny)
library(tidyverse)

# simulate some grouped data
mydata=bind_rows(lapply(LETTERS[1:5],function(x){
  as_tibble(data.frame(xvals=seq(1,100,1))) %>% 
    mutate(grp=x) })) %>% 
  rowwise() %>% 
  mutate(yvals=runif(1)) %>%
  ungroup() %>% 
  group_by(grp) %>% 
  arrange(xvals) %>% 
  mutate(yvals=cumsum(yvals)) %>% 
  ungroup()


ui=fluidPage(
  titlePanel("Dev reactive yaxis"),
  sidebarLayout(
    sidebarPanel(
      radioButtons('graphType',"",c("line","stacked"),"line"),
      uiOutput("ylimits")),
    mainPanel(plotOutput("myplot"))))


server=function(input,output,session) {

  ## y slider
  output$ylimits=renderUI({
    # req(ymax())
    req(input$graphType)
    if(input$graphType=="line"){
      ymax=max(mydata$yvals,na.rm=T)
    } else{
      temp=mydata %>% 
        filter(yvals==max(yvals,na.rm=T))
      ymax=sum(temp$yvals,na.rm=T)
    }
    
    sliderInput('yslider','',min=0,max=ymax,value=c(0,ymax))
  })
  
  ## update slider when graph type changes. This doesn't seem to work :(
  observeEvent(input$graphType=="stacked",{
    # rv$ymax<-funGety
    
    ## get max value for yaxis
     if(input$graphType=="line"){
      ymax=max(mydata$yvals,na.rm=T)
    } else{
      temp=mydata %>% 
        filter(yvals==max(yvals,na.rm=T))
      ymax=sum(temp$yvals,na.rm=T)
    }
    updateSliderInput(session=session,'yslider','',min=0,max=ymax,value=c(0,ymax))
  })
  
  
  ## plot data
  pdat=reactive({
    req(input$yslider)
    mydata %>% 
      filter(yvals>input$yslider[1]&yvals<input$yslider[2])
  })
  
  ## make plot
  myplot=reactive({
    req(pdat())
    req(input$graphType)
    ggplot(pdat(),aes(xvals,yvals))+
      { if (input$graphType=="line") geom_line(aes(colour=grp)) else geom_area(aes(fill=grp)) }
  })
  
  ## render plot
  output$myplot=renderPlot({
    req(myplot())
    myplot()
  })
}

shinyApp(ui=ui,server=server)