These symbols will be used below to coordinate datasets with the same shape. rank of the ``data`` field length of the ``variable`` field length of the ``x`` field length of the ``y`` field length of the ``z`` field .. index:: plotting Declares which dataset is the default. The value is the name of the dataset to be plotted. A field of this name *must* exist (either as dataset or as a link to a dataset). It is recommended (as of NIAC2014) to use this attribute rather than adding a signal attribute to the dataset. See http://wiki.nexusformat.org/2014_How_to_find_default_data for a summary of the discussion. .. index:: plotting String array that defines the independent data fields used in the default plot for all of the dimensions of the *signal* field (the *signal* field is the field in this group that is named by the ``signal`` attribute of this group). One entry is provided for every dimension in the *signal* field. The field(s) named as values (known as "axes") of this attribute *must* exist. An axis slice is specified using a field named ``AXISNAME_indices`` as described below (where the text shown here as ``AXISNAME`` is to be replaced by the actual field name). When no default axis is available for a particular dimension of the plottable data, use a "." in that position. Such as:: @axes=["time", ".", "."] Since there are three items in the list, the the *signal* field must must be a three-dimensional array (rank=3). The first dimension is described by the values of a one-dimensional array named ``time`` while the other two dimensions have no fields to be used as dimension scales. See examples provided on the NeXus wiki: http://www.nexusformat.org/2014_axes_and_uncertainties If there are no axes at all (such as with a stack of images), the axes attribute can be omitted. Each ``AXISNAME_indices`` attribute indicates the dependency relationship of the ``AXISNAME`` field (where ``AXISNAME`` is the name of a field that exists in this ``NXdata`` group) with one or more dimensions of the plottable data. Integer array that defines the indices of the *signal* field (that field will be a multidimensional array) which need to be used in the *AXISNAME* dataset in order to reference the corresponding axis value. The first index of an array is ``0`` (zero). Here, *AXISNAME* is to be replaced by the name of each field described in the ``axes`` attribute. An example with 2-D data, :math:`d(t,P)`, will illustrate:: data_2d:NXdata @signal="data" @axes=["time", "pressure"] @time_indices=0 @pressure_indices=1 data: float[1000,20] time: float[1000] pressure: float[20] This attribute is to be provided in all situations. However, if the indices attributes are missing (such as for data files written before this specification), file readers are encouraged to make their best efforts to plot the data. Thus the implementation of the ``AXISNAME_indices`` attribute is based on the model of "strict writer, liberal reader". .. note:: Attributes potentially containing multiple values (axes and _indices) are to be written as string or integer arrays, to avoid string parsing in reading applications. :ref:`NXdata` describes the plottable data and related dimension scales. .. index:: plotting It is mandatory that there is at least one :ref:`NXdata` group in each :ref:`NXentry` group. Note that the ``variable`` and ``data`` can be defined with different names. The ``signal`` and ``axes`` attributes of the ``data`` group define which items are plottable data and which are *dimension scales*, respectively. :ref:`NXdata` is used to implement one of the basic motivations in NeXus, to provide a default plot for the data of this :ref:`NXentry`. The actual data might be stored in another group and (hard) linked to the :ref:`NXdata` group. * Each :ref:`NXdata` group will define only one data set containing plottable data, dimension scales, and possibly associated standard deviations. Other data sets may be present in the group. * The plottable data may be of arbitrary rank up to a maximum of ``NX_MAXRANK=32``. * The plottable data will be named as the value of the group ``signal`` attribute, such as:: data:NXdata @signal = "counts" @axes = "mr" @mr_indices = 0 counts: float[100] --> the default dependent data mr: float[100] --> the default independent data The field named in the ``signal`` attribute **must** exist, either directly as a dataset or defined through a link. * The group ``axes`` attribute will name the *dimension scale* associated with the plottable data. If available, the standard deviations of the data are to be stored in a data set of the same rank and dimensions, with the name ``errors``. * For each data dimension, there should be a one-dimensional array of the same length. * These one-dimensional arrays are the *dimension scales* of the data, *i.e*. the values of the independent variables at which the data is measured, such as scattering angle or energy transfer. .. index:: link .. index:: axes (attribute) The preferred method to associate each data dimension with its respective dimension scale is to specify the field name of each dimension scale in the group ``axes`` attribute as a string list. Here is an example for a 2-D data set *data* plotted against *time*, and *pressure*. (An additional *temperature* data set is provided and could be selected as an alternate for the *pressure* axis.):: data_2d:NXdata @signal="data" @axes=["time", "pressure"] @pressure_indices=1 @temperature_indices=1 @time_indices=0 data: float[1000,20] pressure: float[20] temperature: float[20] time: float[1000] .. rubric:: Old methods to identify the plottable data There are two older methods of associating each data dimension to its respective dimension scale. Both are now out of date and should not be used when writing new data files. However, client software should expect to see data files written with any of these methods. * One method uses the ``axes`` attribute to specify the names of each *dimension scale*. * The oldest method uses the ``axis`` attribute on each *dimension scale* to identify with an integer the axis whose value is the number of the dimension. .. index: !plot; axis label plot, axis units units dimension scale Each axis of the plot may be labeled with information from the dimension scale for that axis. The optional ``@long_name`` attribute is provided as the axis label default. If ``@long_name`` is not defined, then use the name of the dimension scale. A ``@units`` attribute, if available, may be added to the axis label for further description. See the section :ref:`Design-Units` for more information. .. index: !plot; axis title The optional ``title`` field, if available, provides a suggested title for the plot. If no ``title`` field is found in the :ref:`NXdata` group, look for a ``title`` field in the parent :ref:`NXentry` group, with a fallback to displaying the path to the :ref:`NXdata` group. NeXus is about how to find and annotate the data to be plotted but not to describe how the data is to be plotted. (https://www.nexusformat.org/NIAC2018Minutes.html#nxdata-plottype--attribute) Dimension scale defining an axis of the data. Client is responsible for defining the dimensions of the data. The name of this field may be changed to fit the circumstances. Standard NeXus client tools will use the attributes to determine how to use this field. A *dimension scale* must have a rank of 1 and has length ``n``. Axis label ``0|false``: single value, ``1|true``: multiple values Index of first good value Index of last good value Index (positive integer) identifying this specific set of numbers. N.B. The ``axis`` attribute is the old way of designating a link. Do not use the ``axes`` attribute with the ``axis`` attribute. The ``axes`` *group* attribute is now preferred. "Errors" (actually *uncertainties*) associated with axis ``VARIABLE``. Client is responsible for defining the dimensions of the data. The name of this field may be changed to fit the circumstances but is matched with the *VARIABLE* field with ``_errors`` appended. This pattern of using ``VARIABLE_errors`` can be used throughout a NeXus data file to associate uncertainties with a dataset named ``VARIABLE``. This pattern also applies to other relationships such as ``VARIABLE_resolutions`` to connect additional data with a certain dataset. A dimension scale must have a rank of 1 and has length ``n``, same as ``variable``. .. index:: plotting This field contains the data values to be used as the NeXus *plottable data*. Client is responsible for defining the dimensions of the data. The name of this field may be changed to fit the circumstances. Standard NeXus client tools will use the attributes to determine how to use this field. The rank (``dataRank``) of the ``data`` must satisfy ``1 <= dataRank <= NX_MAXRANK=32``. At least one ``dim`` must have length ``n``. .. index:: plotting Plottable (independent) axis, indicate index number. Only one field in a :ref:`NXdata` group may have the ``signal=1`` attribute. Do not use the ``signal`` attribute with the ``axis`` attribute. Defines the names of the dimension scales (independent axes) for this data set as a colon-delimited array. NOTE: The ``axes`` attribute is the preferred method of designating a link. Do not use the ``axes`` attribute with the ``axis`` attribute. data label Standard deviations of data values - the data array is identified by the group attribute ``signal``. The ``errors`` array must have the same dimensions as ``data``. Client is responsible for defining the dimensions of the data. The ``errors`` must have the same rank (``dataRank``) as the ``data``. At least one ``dim`` must have length "n". The elements in data are usually float values really. For efficiency reasons these are usually stored as integers after scaling with a scale factor. This value is the scale factor. It is required to get the actual physical value, when necessary. An optional offset to apply to the values in data. Title for the plot. This is an array holding the values to use for the x-axis of data. The units must be appropriate for the measurement. This is an array holding the values to use for the y-axis of data. The units must be appropriate for the measurement. This is an array holding the values to use for the z-axis of data. The units must be appropriate for the measurement.