Click the Graphics|New|y(x) Cartesian Graph command to create this kind of graphs. This will cause the y(x) Cratesian Graph dialog box show where you can modify it's properties (formula, color, width and style, intervals and so on).

y(x) Cartesian Graph Dialog box

The Y(x)= box prompts you to enter a function's formula. The right hand side of the formula must be any mathematical expression including constants and an independent variable x.

The Visible option allows you to temporarily hide the graph without deleting it from the project.

The Intervals tab lets you to choose the space for each line. This makes it possible to draw lines that only occupie part of the (x, y)-axes (not the whole as by default).

Use the Color and Size boxes on the right to adjust the color and the thickness of the curve.