See Also
The background of a oneline diagram can display arcs among other items.
Edit Mode
To add a new ellipse, first go to the Draw ribbon tab and select Background > Background Arc from the Individual Insert ribbon group. Then position the cursor where you would like to place the upper left-hand corner of the arc and click with the left mouse button. An ellipse having the default size and shape is inserted. Drag the arc's resizing handles to resize/reshape it.
To resize or reshape an existing arc, click on it to select it. The resizing handles will appear, which you can then drag to resize the arc. The reshape handles will appear inside the resize handles, which you can then drag to reshape the arc.
Right-click on the arc and choose Format Background Ellipse, or select the arc and use the tools in the Formatting ribbon group found on the Draw ribbon tab to change the color, line thickness and fill color of the ellipse.