Created attachment 295604 [details]
simple graphic with filled series

It looks like ods format can do things that xlsx
(not to mention xls) cannot.  Among other things, it
knows about sliders.

However, it does bad things to /filled/ series in an
XY plot.  Recipe:
  Read in the attached file.
  Right-click on the graph;  select Properties.
  Within PlotXY1, select the series called "reflector"
  On the Style tab, change it to Fill-to: Self
  Note that the magenta filled region should have a Γ shape.
  Save
  Quit
  Re-open the file.

Observe that the Fill-to mode is not "Y origin".  The
magenta region has lost its Γ shape.

Desired behavior:  Fill modes should be preserved.

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Note that this has nothing to do with conversion
from gnumeric to ods file format.  It also has
nothing directly to do with interoperation with
other spreadsheet products.  Those are topics for 
another day;  today we are living entirely in 
the ods world.

Note that this is not urgent for me at the moment,
because for present purposes I can restructure my
data so that fill-to-self is not essential.

I haven't thought of an easy way to write an automatic
test for this.  If the recipe were read-then-write, it
would be easy to compare the two files.  In this case,
the only recipe I know to demonstrate the but involves
write-then-re-read, and involves some hand work.  I
hope I am overlooking something;  I hope there is an
easier and more systematic test.