This bugs is perhaps related to this one :
In both cases, the bug (crash or invalid data) arises after the diagram dialog is called and the OK button clicked.
I experimented the same problem as described in the link above. With the help of Stefan Jahn (leader of the qucs project), I tracked down the bug, and it has to do with an unforeseen call to setlocale in /usr/lib/libscim (called from Qt) which changes LC_NUMERIC from "C" to the platfrom language ("FR_fr" in my case).
The problem is that the data files are not correctly read after the change (hence the <invalid> label for the axes, and perhaps crashes in soe cases ?).
Note that the bug appears only if the decimal separator (configured by LC_NUMERIC/LC_ALL) is different from a point (a coma infrench an in other countries).
I join a patch against qucs-0.0.11 which cleared the problem for me. Note it's a "quick and dirty" solution. /usr/lib/libscim sould probably not change the locales during its initialization.