This add-on is to make an axis to help to align objects.

 Download the addon and install it

In edit mode in the menu 'edit' the options appear.

'Axis through plane' - creates an axis at the weighted center of the plane, perpendicular to the plane.

'Axis through three point' - select three points if a circle and an axis is created in the center of  the circle and perpendicular to the circle.

'Cursor at axis' - put the 3d cursor at one of the points of the axis (can be swapped), the direction of the cursor is adapted to the axis direction. If you create a new object at the cursor position and choose 'align- 3D Cursor', the new object is in the direction of the axis.

Be aware that the rotation and scale has to be applied to get the axis and cursor to the right place. In the latest version of the addon there appears a button when the scale or rotation is not applied.

'Remove axis' - removes all axis in this object.