Select the default font family for all legends in the chart. Leaving this field blank will apply the default setting (Arial). Here’s a list of the fonts supported by this visual: 

Arial, Arial Black, Arial Narrow, Agency FB, Brush Script MT, Calibri, Calibri Light, Cambria, Cambria Math, Candara, Comic Sans MS, Consolas, Constantia, Corbel, Courier New, Garamond, Georgia, Helvetica, Impact, Lucida Sans, Papyrus, Segoe UI, Segoe UI Black, Segoe UI Light, Symbol, Times New Roman, Trebuchet MS, Verdana, Wingdings.