If you have not already done so, insert a chart by selecting it from the Insert menu, or double-click an existing chart to access the chart's configuration panel.
Select the Font tab.
From the Font list box, select the type of font for the chart.
From the Style list box, select the style of font (e.g. Plain, Bold).
From the Size list box, select the size of font.
View the result of your selections in the lower half of the window (Sample Text).