Texas Instruments TI-83 PLUS - Manual (Page 224)

TI-83 Plus Draw Instructions 221
Drawing a Line from the Home Screen or a Program
Horizontal (horizontal line) draws a horizontal line at Y= y . y can be an
expression but not a list.
Horizontal y
Vertical (vertical line) draws a vertical line at X = x . x can be an expression
but not a list.
Vertical x
To instruct the TI-83 Plus to draw more than one horizontal or vertical
line, separate each instruction with a colon ( : ).