The compare tool is used to compare each points' value to the first value in a visible range (or to the first point before the range, more info: compareStart).
compare can be enabled in the chart's options, see the API reference. Available options for the
series.compare property are:
percent- percentage difference (image above), e.g. if the first visible value is 10 and the current value is 8, then the compare equals to 80% (or -20% when the compareBase is set to 0).
value- difference between the values, e.g. if the first visible value is 10 and the currect value is 8, then the compare equals to -2.