I want a slice plot of pressure(x, y, z = 0, t) - pressure(x, y, z = 0, t = 0). I tried using a derived field as follows:

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# Load the initial dataset

ds0 = yt.load('my t=0 dataset')

p0 = initialPressure = ds0.all_data()['pressure']

# Define the derived field

def _perturbationPressure(field, data):

        return data['pressure'] - initialPressure

# Add the derived field

yt.add_field(('gas', 'perturbationPressure'), function=_perturbationPressure, units = "auto", dimensions = yt.units.dimensions.pressure)

# Load the dataset of interest

ds = yt.load('my dataset')

yt.SlicePlot(ds, 'z', 'perturbationPressure')

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The error:

 I noticed the "data" in the definition of the derived field is a FieldDetector, so I'm not sure how I should define the derived field to plot the pressure difference (if using a derived field is the right approach at all!). The dataset I'm trying to plot is from FLASH, and I'm running yt 3.6.0. Thanks!