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

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

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()`