Thanks Markkuk and Aleph,
Plot from the plotutils package works.
(Now I have to convince ng-spice not to make the axes prominent and the plot itself in grey dotted lines!) Maybe I shall save the plot output in fig format and hand edit the line types. Unfortunately, the plotted quantity results in a zillion line segments
-- so editing in graphical mode using xfig is out.
I should have reported success earlier -- but had my bandwidth reduced to zero due to deadlines.