This bug is demonstrated by the spreadsheet at
  http://www.av8n.com/computer/graph-fill-bug.gnumeric

Consider the following graph
  http://www.av8n.com/computer/fill-bug-transparent.svg
If we don't pay too much attention to the legend, this can
be considered the graph I am trying to produce.  There is a
black diagonal line, a gray filled band defined by its top 
and bottom, plus a red vertical line.

The legend shows an additional series, "deflect the bug", which
is not readily visible, because its color is almost (albeit not
quite) transparent.

The following graph is similar, except that the "deflect the bug"
series has been made readily visible, to expose the bug and the
workaround:
  http://www.av8n.com/computer/fill-bug-visible.svg

You may be wondering why I am plotting this "deflect the bug" 
series, even though it is not part of the desired result.  Well,
consider the following graph:
  http://www.av8n.com/computer/fill-bug-opacity.svg
This is what happens if I set the opacity of the "deflect the bug"
series all the way to zero.  No other changes have been made.  The
bug manifests itself as follows:  The red curve (which is the fifth
series plotted) gets plotted using the data from the third series,
i.e. the bottom of the gray band.

Similarly, even if the color is fully opaque, if the "deflect the bug"
series has no linecode, the bug attacks the fifth series, as shown here:
  http://www.av8n.com/computer/fill-bug-linecode.svg

For that matter, returning to the cases where the bug is successfully
deflected away from the fifth series, e.g. here:
  http://www.av8n.com/computer/fill-bug-visible.svg
the deflection works by allowing the fourth series to be plotted 
using the data from the third series.  The data intrinsic to the
fourth series (i.e. "deflect the bug") should not produce a trace
that looks like what we see, or indeed anything at all.

Setting a non-blank linecode on the third series (the bottom of the
filled band) is another workaround for this bug.

If I delete the "deflect the bug" series entirely, the same symptom
is observed yet again:  The red curve gets plotted using the data
from the third series.

Note that the .svg images cited here look just the same as the
on-screen graphics produced when gnumeric is running.

This report was prepared using gnumeric version 1.12.1, but
essentially the same misbehavior was observed using 1.11.1.