These plots show information derived from arc observations with a
slit vs. the slit width. 
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The change in spectral resolution is caused by using a more suitable value for
one of
the arc recipe command line parameters since 2017-12. Details are given in the
history
section of the plot tutorial.

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For more information see the
<a href="http://www.eso.org/observing/dfo/quality/XSHOOTER/qc/wavecal_qc1.html">plot tutorial page</a>.