Pls let us know which chart package you're using and provide your chart create's php code. Tks,

We could probably change either the chart's width or its chartArea's right section like this:

ColumnChart::create(array(
    ...
    "options" => array(
        "chartArea" => array(
            "left" => 20,
            "right" => 200, //change this until your legends fit
            // "width" => 500, 
        )
    ),
    // "width" => "1000px"

In our Google chart example, if a legend is cut for being too long we can still see its full value when hovering mouse over. Pls try to comment out all of your options to see if the behavior changes.

Otherwise, your page could have some mouse over event listener or or css hovering property that prevents legends tooltip. One easy way to test it is to make a blank report with only a simple Google chart in it.