Diffraction Grating And Lightbulb

An incandescent lightbulb produces incoherent light. But on Wikipedia, for instance, there is a picture of it producing a rainbow diffraction pattern on the diffraction grating page. But since the bulb is putting out incoherent light, it should be intensities rather than fields that add together.

How is this diffraction from a light bulb possible? For an interference/ diffraction pattern to occur, doesn't the light have to be coherent over the grating's extent? But then how can an incoherent source ever produce diffraction and interference effects?