Drawing Fills in Fill Mode
A fill is a shaded or highlighted area of the canvas. You can use a fill to help emphasize or distinguish particular sections of your graph. To get into Fill mode, tap the Fill tool in the toolbar. (It’s the fat pen drawing the fill on the right.)
To create a fill, drag across the area you'd like to fill, pausing at each corner (but don’t lift!). You need at least three corners, of course, but beyond that you can create as many as you need. When you've created the last corner, lift your finger.
If you draw a fill on top of an existing line or point, the fill and the object snap together. So, subsequently moving the line or point will bring the fill along for the ride.
If two consecutive corners of a fill are part of the same line, the fill conforms to the shape of the line.
Once you’ve created a fill, you can change it with the Fill Style popover.