Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-exists-p(nil)
  org-babel-temp-stable-file(((:cache . "yes") (:colname-names (plotdata "PSO" "" "PPA" "" "GA" "")) (:eval . "no-export") (:exports . "results") (:file . "images/permdbetaevolutionplot.png") (:hlines . "no") (:noweb . "yes") (:result-params "replace" "file") (:result-type . value) (:results . "replace file") (:rowname-names) (:session) (:tangle . "no") (:var plotdata [...] )) "gnuplot-")
  #f(compiled-function (pair) #<bytecode -0x18e177498e7fdfb1> [...])
  org-babel-gnuplot-process-vars([...])
  org-babel-expand-body:gnuplot([...])
  org-babel-sha1-hash([...])
  org-babel-exp-src-block((src-block (:language "gnuplot" :switches nil :parameters nil :begin 68881 :end 69528 :number-lines nil :preserve-indent nil :retain-labels t :use-labels t :label-fmt nil :value "  reset\n  set logscale xy\n  set xlabel \"n_f\"\n  set..." :post-blank 1 :post-affiliated 69077 :name "permdbetaplotevolution" :header (":var plotdata=permdbetaresultstable :file \"images/..." ":results file" ":exports results" ":eval no-export") :mode planning :granularity element :org-element--cache-sync-key (55 . 70525) :cached t :parent (section (:begin 68881 :end 79730 :contents-begin 68881 :contents-end 79679 :robust-begin 68881 :robust-end 79677 :post-blank 51 :post-affiliated 68881 :mode section :granularity element :org-element--cache-sync-key (55 . 70524) :cached t :parent (headline (:raw-value "results" :begin 68810 :end 79730 :pre-blank 0 :contents-begin 68881 :contents-end 79679 :robust-begin 68883 :robust-end 79677 :level 3 :priority nil :tags (...) :todo-keyword nil :todo-type nil :post-blank 51 :footnote-section-p nil :archivedp nil :commentedp nil :post-affiliated 68810 :title "results" :mode nil :granularity element :org-element--cache-sync-key (55 . 68551) :cached t :parent (headline ...))))))))
  org-babel-exp-process-buffer()
  org-export-as(latex nil nil nil (:output-file "book.tex"))
  org-export-to-file(latex "book.tex" nil nil nil nil nil org-latex-compile)
  org-latex-export-to-pdf(nil nil nil nil)
  (org-open-file (org-latex-export-to-pdf nil s v b))
  (if a (org-latex-export-to-pdf t s v b) (org-open-file (org-latex-export-to-pdf nil s v b)))
  (lambda (a s v b) (if a (org-latex-export-to-pdf t s v b) (org-open-file (org-latex-export-to-pdf nil s v b))))(nil nil nil nil)
  org-export-dispatch(nil)
  funcall-interactively(org-export-dispatch nil)
  command-execute(org-export-dispatch)