Determine a run's default plottable data

[prjemian]

Providing content for the plot tab is a bit more of a challenge, requiring analysis of the run's metadata for detectors, motors, hints, and descriptors. This same analysis (to determine the default plottable data) is also needed when writing the data to a NeXus HDF5 file.

Originally posted by @prjemian in https://github.com/BCDA-APS/tiled-viz2023/issues/78#issuecomment-1651854394

[prjemian]

Consider pyqtgraph for plotting

This will be a consideration later when implementing the actual plotting.

[prjemian]

From https://github.com/BCDA-APS/gemviz/issues/111#issuecomment-1670125618:

The first job is to identify what is the default plottable signal and what, if any, are its axes? The terminology in tiled can seem confusing. The NeXus terminology is clearer:

• signal is a data field of a dependent axis
  • there is at most one default plottable
  • that would be the first suggestion
  • user could select a different field from an interface
• axes is a list of data field(s) of the independent axes for the signal
  • the number of axes is the rank of the signal to be plotted
  • there might not be any data to plot as an axis (in which case, use time or index number)
  • there may be more than one data field to use as an axis so need to pick one (e.g., for a $\theta$ - $2\theta$ scan, use $2\theta$)
  • user could select a different field from an interface

[prjemian]

Relevant to #111, the code to identify the plottable data can be derived from the BEC.

[prjemian]

The assumption in BEC is that the plottable data is in the "primary" stream.

[prjemian]

BTW: all of this can be determined by reading the metadata of the run and its primary stream. Should involve minimal additional data transfer from the tiled server.

[prjemian]

Since BEC does not render any images, such as from area detector, we'll need to add that capability.

[prjemian]

Fly scans may write their data into a different stream. That is another consideration for plotting.