Fix standalone flowchart png generation in makefile

book/figures/vars.tex

@@ -1,5 +1,6 @@
 \documentclass[tikz]{standalone}
 \usepackage{tikz}
+\usepackage{pgfplots}
 \usepackage[mode=match, reset-text-family=false]{siunitx}
 \DeclareSIUnit\degF{\text{°}F}
 \input{flowcharts_tikz.tex}

book/makefile

@@ -91,10 +91,9 @@ tgt_figures := $(patsubst %.tex, %.png,$(src_figures))
 # TODO: check if it works on github CI
 %.png: %.tex
 	@echo "\input{./vars.tex}" > $@.in
-	# \b is backspace..
-	@echo "\\\begin{document}" >> $@.in
+	@echo "\begin{document}" >> $@.in
 	@cat $< >> $@.in
-	@echo "\\\end{document}" >> $@.in
+	@echo "\end{document}" >> $@.in
 	$(LATEX) $@.in
 	figures/export_figures.sh $@.pdf