To Fix the Angle of a Line or Ellipse

Select the Constrain Elements tool.
In the tool settings window, set Method to Smart Constrain Elements by selecting the first icon.
Select the line or ellipse.
Enter a second data point on the line or ellipse.
Enter a third data point on the line or ellipse.

Note: To force a line or the primary axis of an ellipse to be horizontal or vertical, set Method to Fix Angle of Line or Ellipse by selecting the fifth icon in the tool settings window and set Angle Lock to Horizontal or Vertical.