To extrapolate a graph, you need to determine the equation of the line of best fit for the graph's data and use it to calculate values for points outside of the range. Alternatively, on simple line graphs, it is sometimes possible to extrapolate from a graph by using a straight edge, like a ruler, to read off a fairly accurate estimate of a nearby point.
Extrapolation is the process of determining likely results based on a set of existing data no matter how far outside either below or above the range of the existing data. Interpolation is a similar process that refers to determining results inside the range of data, but at a specific point where no data is available.
A line of best fit is an imaginary line that goes through the data points on the graph as closely as possible. They can range from simple straight lines to complicated polynomial curves depending on the points on the graph. All lines of best fit have an equation that describes them. For example, a simple line follows the equation y = m*x + b, where "m" is the slope and "b" is the "y" intercept. Once the equation of the line of best fit is known, new values for "x" outside of the existing data range can be substituted in, and new results "y" can be extrapolated through this calculation.