When you have a table with two variables, you can draw a graph (using Excel for instance). In a table about the Dutch population provided bij Statistics Netherlands the two variables $\mathrm{time}$ (year) and $\mathrm{size\; of\; population}$ (× 1000) appear. Here $\mathrm{size\; of\; population}$ depends on $\mathrm{time}$ . Therefore you draw a graph with the independent variable $\mathrm{time}$ on the horizontal axis and the dependent variable $\mathrm{size\; of\; population}$ on the vertical axis.

You denote the names of the variables and their units at the axes as captions. And you use tick marks with numbers on the axis. If you don't do this, you cannot tell the meaning of the axis or the used units. Sometimes a graph gets a graph title. Several types of graphs are denoted:

• in a line graph you connect the points from the table with thin straight lines because you don't know the values in between the points;

• In a curved graph you connect the points from the table with curved lines because you may assume or know that the values in between increase or decrease gradually;

• In a step graph you assume that the dependent variable changes in steps.

Graphs have certain properties...

A (smooth) graph is called

• increasing if the dependent variable increases when the independent variable increases;

• decreasing if the dependent variable decreases when the independent variable increases.

The graph has

• a maximum when it goes from increasing to decreasing;

• a minimum when it goes from decreasing to increasing.

You often find a maximum or minimum at the edges.
The maximum and minimum values are called extreme values.

Sometimes the dependent variable repeats its behaviour over a certain period.
We call such a graph that (kind of) repeats itself periodical.
This graph of a normal heart rhytm is an example of a periodical graph. The period is approximately $\mathrm{0,8}$ seconds. That means a frequency of $75$ heart beats per minute.