This is just a work-around tip to draw a straight (vertical or horizontal) line :

1) Draw a rectangle by Box tool button;
2) Double click it to show the Shape Control dialog. Then Change the border color to Black, and make the Fill Color check box ON. Most people may want to change the Thickness to 2.0, otherwise the line gets thicker;
3) Using the mouse, change the shape by making the hight (for horizontal line) or the width (for vertical line) zero.