Working in highcharts where the years will be selectable….I want to include a % change from the other year in the tooltips.

Test data:

test <- tribble(
  ~Year,    ~Type,    ~Total,    ~Median,
  1995,      'A',       20198,       1873,
  1995,      'B',        4589,        324,
  1995,      'C',       12984,       1209,
  1996,      'A',        3829,       2091,
  1996,      'B',        3829,       3045,
  1996,      'C',        3829,       9291,
  1997,      'A',       14023,       1829,
  1997,      'B',       16093,       2129,
  1997,      'C',        1893,        529
)

Calculate percent change between the two years

   percent_change <- function(year2_fig, year1_fig) {

((year1_fig - year2_fig) / year2_fig) * 100

}

How to refer to the the mouse selector position on the chart to feed in variables to this percent change function?

# --- create tooltip table ----
tooltip_category_text <- c("Median:  ",  "Percent change: " )
tooltip_formatted_values <- c(" {point.Median}",  "{point.percent_change()}")

my_tooltips <- tooltip_table(tooltip_category_text, tooltip_formatted_values)

Chart

highchart() %>% 


hc_chart(type = "column",
           marginLeft = 200) %>% 
  hc_plotOptions(series = list(pointWidth = 18)) %>% 
  hc_yAxis(labels = list(style = 
                           list(fontFamily = "Segoe UI",
                                fontWeight = 100), enabled =TRUE)) %>% 
  hc_xAxis(categories = test$Type,
           labels = list(style = 
                           list(color = '#344a5e',
                                fontFamily = "Segoe UI",
                                fontWeight = 100,
                                fontSize = '12px'), enabled = TRUE)) %>% 
  hc_add_series(dataSorting = list(enabled = TRUE),
                data = test, type = 'bar', hcaes(x = Type, y = Total, group = Year),
                dataLabels = list(enabled = FALSE, style = list(color ='#344a5e',
                                                                fontFamily = "Segoe UI",
                                                                fontWeight = 200))) %>% 
  hc_tooltip(
    pointFormat = my_tooltips,
    useHTML = TRUE
   )

Have seen the method to use a Javascript formatter but again cannot reference a function within this…

  hc_tooltip(
    formatter = JS("function(){return ('Type: ' + this.point.Type +
                 '<br> Median : ' + this.point.test$Median ) + 
                 '<br> Percent Change: ' + this.point.percent_change(year2, year1)}")

This tooltip formatter is only working with the first point, why not the second, and what way can I include the third?