R编程高级字符-在工具提示中减去两个值

Question

我在R中又遇到了一个高级章程问题。在堆叠的条形图中有两个值，我想通过JS函数减去这些值。但是，由于某些值也为负，因此无法正常工作。我不知道该如何处理..

bs.table = data.frame(
  Closing.Date = c("Year 2012", "Year 2013", "Year 2014", "Year 2015", "Year 2016"),
  Non.Current.Assets = c(40.4, 30.3, 20.4, 34.5, 20),
  Current.Assets = c(3.2, 3.3, 2.4, 3.5, 2)
)
hc <- highchart() %>% 
  hc_chart(type = "bar") %>%
  hc_plotOptions(series = list(stacking = "normal")) %>%
  hc_xAxis(categories = bs.table$Closing.Date,
      lineColor = 'transparent',
      tickWidth = 0,
      labels = list(enable = TRUE, 
                     align = 'left',
                     x = 5,
                    style = list(fontSize = '1em',color = '#fff'))) %>%
  hc_add_series(name ="Non Current Assets",
            data = bs.table$Current.Assets,
            stack = "Assets",
            dataLabels = list(enabled = TRUE, 
                              inside = TRUE, 
                              align = "right", 
                              style = list(fontSize = '1em',color = '#fff'))) %>%
  hc_add_series(name = "Current Assets",
            data = bs.table$Non.Current.Assets,
            stack = "Assets",
            dataLabels = list(enabled = TRUE, inside = FALSE, align = "right", 
                              style = list(fontSize = '1em',color = '#fff')) ) %>%
  hc_legend(enabled = FALSE) %>%
  hc_tooltip(shared = TRUE,
          formatter=JS("function() {var s = '<b>'+ this.x +'</b>',sum = 0;$.each(this.points, function(i, point) {s += '<br/>'+ point.series.name +': '+point.y;sum -= point.y;});s += '<br/>Sum: '+sum;return s;}"))  
  hc

任何帮助将不胜感激！