Highcharts/HighcharteR Tooltip: access all y-values within a series, print the difference

Question

[EDIT] I ended up solving it as I was writing this question, so see below for the answer.

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I have a graph that has time periods on the x-axis, and scores on the y-axis, and at each point I'd like the tooltip to give the difference in y values between the current point and the previous one (within the same series).

Reproducible example:

library(highcharter)

hchart(df, 
       type="line", 
       hcaes(x = period, y = value, group = group)
  ) %>%
  hc_tooltip(pointFormat = "Score: {point.y} ({previous.point.y})")

Ideally, on hovering for example over the second point for series B, I'd like it to say Score: 5 (+1). This will probably require some formatter=JS() JavaScript instead of just pointFormat, but not sure how to do that.

Answer 1

Courtesy of this related answer, I managed to access all y values, after which I figured out that using this.point.x allows us to hone in on specific y values. Here's the JS:

function () {
  if (this.point.x == 0) {  // there's no previous point, so set to '0'
    var thisDiff = 0;
  } else {  // use 'this.point.x' to get current position, and do '-1' to get previous position
    var thisDiff = ( this.series.points[this.point.x].y - this.series.points[this.point.x - 1].y );
    if (thisDiff > 0) {
      thisDiff = '+' + thisDiff;  // pretty print a '+' sign if difference is positive
    }
  }
  var s = '<b>Series ' + this.series.name + ', period ' + this.point.name + ': </b>';
  s += 'mean score ' + this.point.y + ' (' + thisDiff + ')';
  return(s);
}

To get it to work in highcharteR we just need to wrap this in quotation marks and pass to hc_tooltip(formatter = JS()