(noun) A line graph can be used to show how something changes over time. They have an x-axis (horizontal) and a y-axis (vertical). Usually, the x-axis has numbers for the time period, and the y-axis has numbers for what is being measured.
(n) A visualization technique, usually 2-D, in which line marks are used to indicate trends in data. In a 2-D line graph, the independent variable is represented on the horizontal scale, and the dependent variable is shown on the vertical scale. The line mark represents the mapping of the two variables.