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Volumetric Flow Rate

I have a set of pressure measurements at various points along a horizontal free-flowing air jet. The jet exits through a nozzle, then the pressure at the nozzle is measured. At 60, 180 and 300mm away from the nozzle, pressure measurements are made at 5mm vertical intervals out to 60mm each side. This means I have graphs showing velocity across the whole flow at each point.

How do I calculate the volumetric flow rate at each point? The flow isn't uniform, so I should integrate across the whole flow - but I don't have an equation, I have a set of discrete data. What numerical method could I use?

I've tried using a form of the trapezium rule (connecting each data point with a straight line) combined with a volume of revolution, but I don't know if that's right.

I think that might be wrong, because at the nozzle I get a flow rate of about 12Litres/sec, but at 60mm it's 33mL/sec, when it should be roughly the same due to conservation of energy.

Anyone have any ideas? Sorry for the long post!