This plot shows 

<ul>
<li> the number of identified arc lines in (RAW_TYPE = FMTCHK, recipe =
xsh_predict, 1 single-pinhole) frames
<li> the number of identified arc lines in (RAW_TYPE = WAVE, recipe =
xsh_2dmap,  1 9-pinhole arc) frames
<li> the number of identified orders (RAW_TYPE = ORDERDEF, recipe =
xsh_orderpos, 1 flat and 1 single-pinhole) frames
</ul>

More information can be found in the
<a href="http://www.eso.org/observing/dfo/quality/XSHOOTER/qc/distortion_qc1.html">plot tutorial page</a>.