How do I send multiple dySeries to a secondary axis based on checkbox input in shiny?

I'm trying to plot a dyGraph using the lungDeaths timeseries, but I want "mdeaths" and "fdeaths" to be on the secondary axis, if at least one of them is selected alongside "ldeaths".

Here's a working example:

global.R

library(dygraphs)
library(shiny)
library(datasets)
library(stringr)

lungDeaths <- cbind(mdeaths, fdeaths)

ui.R

shinyUI(fluidPage(

titlePanel("Predicted Deaths from Lung Disease (UK)"),

sidebarLayout(
    sidebarPanel(

        id="sidebar", width = 3,
        div(
            checkboxGroupInput(
                inputId = "selection", label = "Variables:",
                choiceNames = list("ldeaths",
                                   strong("mdeaths"),
                                   strong("fdeaths")
                                   ),
                choiceValues = c("ldeaths",
                                 "mdeaths",
                                 "fdeaths"), selected = "ldeaths"),
            uiOutput("rendered"),
            style = "font-size:75%"
        )

    ),
    mainPanel(
        dygraphOutput("dygraph")
    )
)

))

server.R

shinyServer(function(input, output) {

lungDeaths <- cbind(mdeaths, fdeaths, ldeaths)

rData <- reactive({
    rData <- ts(lungDeaths[,input$selection])
})

output$dygraph <- renderDygraph({

    if(length(input$selection) > 1 & length(str_subset(input$selection, 'ldeaths$'))>0){

        if(length(str_subset(input$selection, 'fdeaths$'))>0 & length(str_subset(input$selection, 'mdeaths$'))>0){
            dygraph(rData(), main = "Deaths from Lung Disease (UK)") %>% 
                dySeries("mdeaths", axis = 'y2') %>% 
                dySeries("fdeaths", axis = 'y2')
        }

        else if(length(str_subset(input$selection, 'mdeaths$'))>0){
            dygraph(rData(), main = "Deaths from Lung Disease (UK)") %>% 
            dySeries("mdeaths", axis = 'y2') 
        }
        else if(length(str_subset(input$selection, 'fdeaths$'))>0){
            dygraph(rData(), main = "Deaths from Lung Disease (UK)") %>% 
            dySeries("fdeaths", axis = 'y2')

        }
    }
    else
        dygraph(rData(), main = "Deaths from Lung Disease (UK)")
})

})

This code does what I intend it to do, but I wanted to avoid using so many if's and else's, because the project I'm actually working with has 6 items that should go to y2 when selected. Is there any way to do it without having to specify each possibility?

I've already researched on how to add dySeries based on input and how to do conditional evaluation when using pipes, but the answers I found so far don't work for me. I usually get an error that says:

$ operator is invalid for atomic vectors