Error(s) in re-building vignettes:
--- re-building ‘graphviz-mermaid.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
Quitting from lines 57-79 (graphviz-mermaid.Rmd) 
Error: processing vignette 'graphviz-mermaid.Rmd' failed with diagnostics:
cannot open the connection
--- failed re-building ‘graphviz-mermaid.Rmd’

--- re-building ‘node-edge-data-frames.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
--- finished re-building ‘node-edge-data-frames.Rmd’

--- re-building ‘selections.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
Quitting from lines 317-356 (selections.Rmd) 
Error: processing vignette 'selections.Rmd' failed with diagnostics:
cannot open the connection
--- failed re-building ‘selections.Rmd’

--- re-building ‘simple-graphs-ndfs-edfs.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
Quitting from lines 165-166 (simple-graphs-ndfs-edfs.Rmd) 
Error: processing vignette 'simple-graphs-ndfs-edfs.Rmd' failed with diagnostics:
cannot open the connection
--- failed re-building ‘simple-graphs-ndfs-edfs.Rmd’

--- re-building ‘traversals.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.
Quitting from lines 102-116 (traversals.Rmd) 
Error: processing vignette 'traversals.Rmd' failed with diagnostics:
cannot open the connection
--- failed re-building ‘traversals.Rmd’

SUMMARY: processing the following files failed:
  ‘graphviz-mermaid.Rmd’ ‘selections.Rmd’ ‘simple-graphs-ndfs-edfs.Rmd’
  ‘traversals.Rmd’

Error: Vignette re-building failed.
Execution halted