Found it! Using tooltipFormatter, I did something like this:
sparkOpts.CurrentTimeGroup = currentTimeGroup;
sparkOpts.tooltipFormatter = function(sparklines, options, point)
{
if(point.x <= options.mergedOptions.CurrentTimeGroup)
return "<div class=\"jqsfield\"><span style=\"color: " + point.color + "\">●</span>" + options.get("tooltipValueLookups").names[point.x] + " - " + point.y + options.get("tooltipSuffix") + "</div>";
else
return "<div class=\"jqsfield\"><span style=\"color: " + point.color + "\">●</span>" + options.get("tooltipValueLookups").names[point.x] + "</div>";
};