Measuring the volume of a gaseous product in a chemical reaction is very common. The gas is collected in a gas syringe or by displacement of water in an upturned measuring cylinder.

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Readings are taken of the volume of gas produced in a set unit of time. For the reaction described in the diagram, readings are taken from the measuring cylinder every 2 minutes. This generates a graph that looks like this:

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The slope of the line is changing over time. Where it is steepest the rate is high, and where it levels out the reaction has stopped. With a graph like this it's possible to see that the rate is not constant over the duration of the experiment.

More support can be found on the following pages of Maths skills online course. Using graphs: linear relationships and Using graphs: non-linear relationships: example 2.