Take into account that the domain are the set of x values with point on the line.
Furthemore, consider that for a piecewise function, blank points means that such a point is not included in the domain and range.
As you can notice, there is a blank point at (-1,-1) according to the scale of the coordinate system. The x-coordinate of this point is -1.
Then, -1 is not in the domain.
