Many graphs are made from tables with measurement data. The way you connect the measurements is your choice. In a line graph you generally connect them by straight lines, but sometimes you use a smooth curve. You can only estimate the values that are not in the table.

• interpolation is the estimation of values between two known values. This is often done bij drawing a straight line through two points and reading off the value in between (mid 2002 there were approximately 870.000 people);

• extrapolation is the estimation of values outside the measurement area. This is often done by drawing a straight line through the two previous (or the two following) measurements and extending it and reading off the desired value (early 2008 approximately 968.000 people).

Sometimes you encounter more than one graph with the same variables on the axes. This is called a bundle of graphs. From these you can read off values that do not exist in the corresponding tables, even in the area between two graphs.