Convert data to class breathtestfit — null

Does not change the data set, but returns a class suitable for plotting raw data with plot.breathtestfit. See read_any_breathtest for an example.

Usage

null_fit(data, ...)

data: Data frame or tibble as created by cleanup_data, with mandatory columns patient_id, group, minute and pdr.
...: Not used

A list of classes breathtestnullfit, breathtestfit

with element data which contains the unmodified data.