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.
Access the Chart tab.
In the Summary Line section, select the Show check box to enable the summary line.
From the Type list, choose the type of data/calculation you want plotted as a summary line.
From the Line Style list, choose the type of line you want drawn between points.
If you want to change the color of the line, click the Color box and choose a new color from the palette. Click OK.
In the Line Width box, enter a number that represents the width (in pixels) of the summary line. 1 is the narrowest line; 9 is the thickest line.
From the Marker Style list, choose a symbol to be shown for the plotted points of the summary line.
If you want to change the color of the marker, click the Marker Fill Color box and choose a new color from the palette. Click OK.