dynamically plot multiple variables in shiny dashboard R

I am trying to build a shiny dashboard that shows the user a plot of data over time for multiple countries and multiple variables. The dashboard works pretty well, but I would really appreciate some help in overcoming a problem I've been having:

• I am currently able to plot multiple countries for a single variable (e.g. Variable_1 for USA and AFG over some time period)

• However, I am currently unable to plot multiple countries for multiple variables (e.g Variable_1 and Variable_2 for USA over some time period OR Variable_1 and Variable_2 for USA and AFG over some time period). When I try and select multiple variables, I receive the error "more than one expression parsed".

I would very much appreciate a method of being able to generate ggplots of multiple variables without hard coding since the actual data contains many more variables and many more countries. If anyone could point me in the right direction or provide a solution, that would be fantastic. A minimal working example is provided below.

Thankyou to anyone who is able to help! :)

library(shiny)
library(dplyr)
library(tidyverse)

df = data.frame("Country" = c("USA","USA","USA","USA","AFG","AFG","AFG","AFG"),
                "Year" = c(2000,2001,2002,2003,2000,2001,2002,2003),
                "Variable_1" = c(10,12,14,16,10,11,12,13),
                "Variable_2" = c(20,19,18,17,20,21,22,23),
                "Variable_3" = c(13,13,14,14,16,16,12,12))

#df_long <- melt(df, id=c("Country","Year"))  

ui = fluidPage(
  titlePanel("My Dashboard"), 
  
  pickerInput("myvariable","Pick variables", choices =c("Variable_1","Variable_2","Variable_3"),
              options =list("actions-box" = TRUE),
              multiple=TRUE, 
              selected = "Variable_1"),
  
  sliderInput("year_selector", "Select Year Range",min = 2000,max = 2003,value = c(2000, 2013)),
  
  pickerInput("choicePicker","Pick countries",choices =c("USA", "AFG"),
              options =list("actions-box" = TRUE), 
              multiple=TRUE,
              selected="USA"),
  
  plotOutput("trend")
    )
  
server = function(input, output, session){
  selected <- reactive(filter(df, Country %in% input$choicePicker, Year>=input$year_selector[1], Year<=input$year_selector[2]))
  output$trend = renderPlot({
    ggplot(selected(), aes_string(x="Year", y=input$myvariable, color="Country", group="Country")) +
      geom_line(size = 2) +   
      scale_x_continuous(breaks = pretty_breaks()) +
      labs(x = "",
           y = paste0(input$myvariable),
           title = paste(" ,", " "),
           caption = paste(" ", " ")) +
      theme(legend.position = c(0.8, 0.8))
  })
}

shinyApp(ui=ui, server=server)