Lines that Parallel the x and y Axes
Some linear functions are known as constant functions. They are characterized by vertical or horizontal lines that run parallel to the y or x axes. These functions assume one of two forms. In the first form, the value of y remains the same regardless of the position you plot relative to the x axis. This line runs parallel to the x axis and is distinguished only by the changing value of x.
y = a
In y = a, a identifies a constant value. When you plot such a function, the graph assumes the form shown in Figure 10.8. You equate the lines this figure shows as a flat slope. In other words, the slope has no rise, only a run. For all values of x, y remains the same.
To use Visual Formula to plot constant values, click on the Value menu item. Then click in the upper equation composition area to position the Value field. Type 5 in the field. The equation you implement takes the form of y = a, as is illustrated in Figure 10.8. Click the Chart for Formula 1 button to view the graph. Figure 10.9 illustrates the result.
To generate the lower line in the Cartesian plane, click the Value menu item. Then click in the lower of the two equation composition areas. In the field that results, type –3. Then in the lower-right panel, Click Chart for Formula 2. As Figure 10.9 illustrates, the lower line is beneath and parallel to the x axis.
To set Visual Formula so that the lines and scales resemble those shown in Figure 10.9, type –20 in the From field beneath the X Range label for the Chart for Formula 1 button. In the To field, type 20. In the Chart panel, set the Zoom values to –3. Set the Width fields to 2. If you click the Color fields, you can choose the colors you find preferable from the color palette.