组合表头
通过 Grid::combine
方法可以很方便的把任意两个以上的字段组合成一级表头

示例
protected function grid()
{
return Grid::make(new Report(), function (Grid $grid) {
// 第一个参数为一级表头字段名称,第二个字段为二级表头字段名称,二级表头字段最少设置两个
$grid->combine('avgCost', ['avgMonthCost', 'avgQuarterCost', 'avgYearCost']);
$grid->combine('avgVist', ['avgMonthVist', 'avgQuarterVist', 'avgYearVist']);
// 设置样式
$grid->combine('top', ['topCost', 'topVist', 'topIncr'])->style('color:#1867c0');
$grid->content->limit(50);
$grid->cost->sortable();
$grid->avgMonthCost;
$grid->avgQuarterCost->setHeaderAttributes(['style' => 'color:#5b69bc']);
$grid->avgYearCost;
$grid->avgMonthVist;
$grid->avgQuarterVist;
$grid->avgYearVist;
$grid->incrs->hide();
$grid->avgVists->hide();
$grid->topCost;
$grid->topVist;
$grid->topIncr;
$grid->date->sortable();
});
}