Draw Lines and Line Segments on a TI83 Plus Graph
With the TI83 Plus graphing calculator, you can draw line segments, horizontal lines, and vertical lines on the graph of functions, parametric equations, polar equations, or sequences. To draw a line segment on your graph, follow these steps:

Graph the functions, parametric equations, polar equations, or sequences.

Press [2nd][PRGM][2] to select the Line option from the Draw menu.
The coordinates of the cursor appear at the bottom of the screen, provided that CoordOn is highlighted in the Format menu.

Use the
keys to move the cursor to the location of one endpoint of the segment and press [ENTER].

Use the
keys to move the cursor to the location of the other endpoint of the segment and press [ENTER].

Repeat Steps 3 and 4 to draw another segment or press [CLEAR] when you’re finished drawing line segments.
The procedures for drawing horizontal and vertical lines are similar to the procedure for drawing a line segment. To draw a horizontal line, perform the following steps. (For a vertical line, you would select the Vertical option from the Draw menu in Step 2. Otherwise, the procedures are the same.)

Graph the function, parametric equations, polar equation, or sequence.

Press [2nd][PRGM][3] to select the Horizontal option from the Draw menu.

Use the
keys to move the cursor to the location of the horizontal line and press [ENTER].
If using the
keys doesn’t take you to the exact location of your horizontal line, press [2nd][MODE] to exit the graph. Then press [2nd][PRGM][3], enter the ycoordinate of the horizontal line, and press [ENTER]. Your horizontal line appears on the graph.

Repeat Step 3 to draw another horizontal line or press [CLEAR] when you’re finished drawing horizontal lines.
If you used the Tip in Step 3 to draw the horizontal line, then you won’t have to press [CLEAR] when you’re finished. But you must press [2nd][PRGM][3] again if you want to draw more horizontal lines.