When all the plugins have been disabled, adding a plot to a chart in the guru
crashes gnumeric. We need some kind of dialog warning to catch this.


1) new gnumeric
2) Tools -> Plugins -> Deactivate all
3) click on graph guru button
4) click ok, add sheet object to sheet
gdb gives:
** (gnumeric:7555): CRITICAL **: file gog-guru.c: line 214
(graph_typeselect_minor): assertion `plot != NULL' failed


5) context menu => properties
6) add a plot to the chart
==> crash

gdb gives:
(gnumeric:7555): GLib-GObject-WARNING **: cannot create instance of abstract
(non-instantiatable) type `GogPlot'

Program received signal SIGSEGV, Segmentation fault.

+ Trace 50592