xl help file:
Change the default font and font size used in new workbooks
On the Tools menu, click Options, and then click the General tab.
In the Standard font box, click a font.
In the Size box, enter a font size.
Note You must restart Microsoft Excel to begin using the new default font and font size. The new default font and font size are used only in the new workbooks you create after you restart Microsoft Excel; existing workbooks are not affected.
Try exiting xl, restart and create your charts then in a new work book