bug fix: plot(dump=True) works with complex terminal strings

There are two bugs here. This patch fixes only the breakage we get with
dump=True. Prior to this patch we'd try to access self.fdDupSTDOUT even when it
doesn't exist. The next patch will fix the incomplete terminal detection

Author: dkogan

gnuplotlib.py

@@ -2346,10 +2346,12 @@ def plot_process_header():
                         self._safelyWriteToPipe('set output',
                                                 'output')
                     else:
-                        if self.fdDupSTDOUT is None:
-                            raise GnuplotlibError("I need to plot to STDOUT, but STDOUT wasn't available")
-
-                        self.processOptions['output'] = '/dev/fd/' + str(self.fdDupSTDOUT)
+                        if not _data_dump_only(self.processOptions):
+                            if self.fdDupSTDOUT is None:
+                                raise GnuplotlibError("I need to plot to STDOUT, but STDOUT wasn't available")
+                            self.processOptions['output'] = '/dev/fd/' + str(self.fdDupSTDOUT)
+                        else:
+                            self.processOptions['output'] = '/dev/fd/DUMPONLY'
                         self._safelyWriteToPipe('set output "' + self.processOptions['output'] + '"',
                                                 'output')