Extrapolate: to extend the line (usually of a graph) to approximate or predict values outside the range of the graph.
In this case pupils are required to extend the graph to approximately determine the birth weight of the joey. To do this they extend the line of the graph until it intersects the y-axis. 0 days on the x-axis represents the birth of the joey, the pint on the y-axis corresponds to the weight of the joey at birth.