From the View menu, Show Rulers. Now, from the top of the horizontal ruler line, drag a single guide down to be your baseline for your first curved line. I started by clicking the Connection line shape which gives you this canted default:

Just click on the right (top) grip and drag downward to align both end-points on the guide:

Click on the curve to select it, and then in the Arrange panel, set Text Wrap to None. You can now change the height and spread of the curve by experimenting with the x-coordinate start and end increment buttons. You can also duplicate the curve via either ⌘-D, or option+click+drag. If I wanted to create a sine-wave, I would take the duplicate, and from the Arrange panel, click the up-arrow version of flip. Then drag the inverted curve below the guide, and use the arrow keys to nudge it into alignment with the top curve. Go back to the first curve and duplicate it, then drag across the guide until you can align it with the bottom curve.

Unfortunately, this is done in Pages v8.2 and there is neither a way to save this back into Shapes for future usage, nor is there a way to get this out of Pages in other than a low resolution, 72-dpi PNG image via copy/paste and Preview. The smart money is doing this kind of artwork in an external vector tool such as Affinity Designer, where you can start at 300 dpi, have better curve options, and export to a wide variety of image and PDF formats for future reuse of high resolution vector artwork.