ClarisWorks 1.0
The ClarisWorks 1.0 Handbook states that you can change the line weight of a chart by clicking on the legend element and choosing a new line weight from the palette. This does not work. You can change color and pattern but not line width.
The work-around is to copy the chart and paste it back in as a graphic object. Each line becomes a selectable object and line weights can be modified. However, the chart no longer maintains its link to the spreadsheet data.
ClarisWorks 2.0
ClarisWorks 2.0 adds the capability to modify chart line weights. However, only line weights of 1 point and larger are supported. Changing a line to Hairline will cause it to switch to None instead.