If you want to make style changes to elements of a chart created in the spreadsheet environment you might refer the section "To create a chart" of the ClarisWorks Handbook which states, "Any changes you make to the font, size, or style in the spreadsheet appear automatically in the chart." This feature is quite apparent when changing the style of the category series or the legend.
However, if you want to change the font, size, style, etc. of the number series, you need to make the desired changes to the top left cell (anchor cell) of the range of numbers used to create the chart.
For example, you might have a range of data from cell A1 to D4, where row 1 is used for the chart categories and the numerical data for the chart is in the range B2...D4. The cell B2 represents the anchor cell for the data used in number series of the chart and any style changes made to this cell will automatically be reflected in the chart. Changing the style of any other cell in the range B2...D4, even if every cell in that range except B2 is changed, will not be reflected in the number series of the chart.