This plot monitors the shutter issue, that occured  between 2015-09-09 and 2016-06. 
The issue is explained on the SPHERE problems page and on the plot tutorial
page for zimpol flats. 
Only the polarimetric direction = zero and cycle  = odd signal is monitored. 
The following setups are covered:

<ul>
<li> V filter,   DIT, NDIT = 4.0, 10
<li> N_R filter, DIT, NDIT = 2.0, 20
<li> N_I filter, DIT, NDIT = 2.0, 40
</ul>
The plot contains four boxes, which shows the shutter parameter (essentially
the RMS over the NDIT subintegration signals) 
for both read modes (SlowPolarimetry and Fastpolarimetry) and for both
detectors.