[Nexus] Correct way to specify multiple signals

Osborn, Raymond rosborn at anl.gov
Thu Jan 11 14:36:47 GMT 2018

I saw the Github issue and responded before I realized there was already a lengthy correspondence here. I will have to read more carefully what has already been contributed here, but I wanted to quickly summarize what I wrote there (https://github.com/nexusformat/NIAC/issues/25):

“In my view, it is the role of the NeXus standard to provide a way to offer a default plot for each NXdata group, but I don't think it is necessary for NeXus to then specify all other possible non-default plots. Any program is free to provide that option within the current standard as NeXpy does.”

Please see the issue for my reasoning - basically, you can do what Armando requests, i.e., plot multiple signals, now without adding extra complexity to the standard.

In recent years, I have become concerned about the standard becoming more complicated than it need be, partly because of my experience in writing a generic NeXus application. Every new layer of complexity makes the files increasingly difficult to parse (as well as more complex to write), and so I would recommend that we only specify what is truly necessary.


On Jan 11, 2018, at 8:01 AM, V. Armando Solé <sole at esrf.fr<mailto:sole at esrf.fr>> wrote:

On 11/01/2018 14:50, Peter.Chang at Diamond.ac.uk<mailto:Peter.Chang at Diamond.ac.uk> wrote:
My suggestion would be an auxiliary attribute in the NXdata group to point to the dataset(s) to be optionally plotted alongside the signal dataset:

@signal: fitted
@auxiliary: [raw_data]

Quite smart way. I like it!!!

It is so flexible that it would even allow to be generalized as
auxiliary_signals, auxiliary_axes, so that even the condition of sharing
the same axes is not needed and one would be plot, for instance, the raw
data and the fitted data with different number of points (a simple use
case: with more points on the fitted data to better visualize a gaussian
shape if required). Current programs would work and everything would be

NIAC guys, what about *extending* NXdata in these lines ?

A default plot/display type would be handy as well as the plot dimensions to use:

@display: line
@display_dimensions: [-1] (last dim as default)


@display: image
@display_dimensions: [-2,-1] (last two dims as default)

On those I disagree. To be images or spectra is an intrinsic property of
the dataset, not a plot property.


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Ray Osborn, Senior Scientist
Materials Science Division
Argonne National Laboratory
Argonne, IL 60439, USA
Phone: +1 (630) 252-9011
Email: ROsborn at anl.gov<mailto:ROsborn at anl.gov>

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