Return the data used by plot_data.
Parameters: id (int or str, optional) – The data set that provides the data. If not given then the default identifier is used, as returned by get_default_id. Returns: data – An object representing the data used to create the plot by plot_data. The relationship between the returned values and the values in the data set depend on the data type. For example PHA data are plotted in units controlled by sherpa.astro.ui.set_analysis, but are stored as channels and counts, and may have been grouped and the background estimate removed. Return type: a sherpa.plot.DataPlot instance