Final answer:
A straight vertical line on a graph is called a constant line, or a line with a slope of zero. This means that there is no relationship between the x and y values.
Step-by-step explanation:
A straight vertical line on a graph is called a constant line, or a line with a slope of zero. This means that there is no relationship between the x and y values. The line is flat, showing zero rise over the run. For example, a line graph of displacement versus time for a jet-powered car on a flat dry lake bed would show a straight vertical line.