The HTML of the point's line in the tooltip. Variables are enclosed
by curly brackets. Available variables are point.x, point.y,
series.name and series.color and other properties on the same
form. Furthermore, point.y can be extended by the
tooltip.valuePrefix and tooltip.valueSuffix variables. This can
also be overridden for each series, which makes it a good hook for
displaying units.
In styled mode, the dot is colored by a class name rather than the point color.