Hope that you are able to download new version of ChartJS that I sent to you through email. Please following below code to make the effect that you want:
<?php
ColumnChart::create(array(
    "title"=>"Sale Report",
    "dataSource"=>$category_amount,
    "columns"=>array(
        "category",
        "sale"=>array(
            "label"=>"Sale",
            "type"=>"number",
            "prefix"=>"$"
        ),
        "cost"=>array(
            "label"=>"Cost",
            "type"=>"number",
            "prefix"=>"$"
        ),
        "profit"=>array(
            "label"=>"Profit",
            "type"=>"number",
            "prefix"=>"$",
            "config"=>array(
                "type"=>"line",
                "borderWidth"=>3,
            )
        ),
    ),
    "options"=>array(
        "tooltips"=>array(
            "mode"=>"index",
            "intersect"=>true,
        )
    )
));
?>
Please pay attention to the tooltips in options