There are several ways to approach this:
One way is to specify separate series text and legend item text using the
legendText attributes, respectively, in the individual series objects. See the below chart.
Another way is to set the maximum number of characters you want displayed by adding a
maxChars attribute to the
item object in your legend. You can also add legend tooltips with the
tooltip object to display the full series name when users hover over the legend items. See the below chart.
Refer to the Legend Tutorial for more information on the various legend elements and interactive legend features.