`generate_dot()` uses single quotes instead of double quotes

[atchley-sha]

Using the generate_dot() function, the output uses single quotes around every attribute value, which (as far as I can tell) isn't supported in the dot language. Writing the output of generate_dot() to a file and subsequently running dot from a command line on that file throws a syntax error.

For example:

g <- create_graph() %>%
    add_node() %>%
    add_node() %>%
    add_edge(1,2)

generate_dot(g) %>% writeLines("test.dot")

produces

digraph {

graph [layout = 'neato',
       outputorder = 'edgesfirst',
       bgcolor = 'white']

node [fontname = 'Helvetica',
      fontsize = '10',
      shape = 'circle',
      fixedsize = 'true',
      width = '0.5',
      style = 'filled',
      fillcolor = 'aliceblue',
      color = 'gray70',
      fontcolor = 'gray50']

edge [fontname = 'Helvetica',
     fontsize = '8',
     len = '1.5',
     color = 'gray80',
     arrowsize = '0.5']

  '1'
  '2'
  '1'->'2' 
}

and dot -Tpng test.dot -o test.png from a command line gives Error: test.dot: syntax error in line 3 near '''. However, replacing the single quotes with double quotes (e.g. with str_replace("'", "\"")) produces a file that does work with command-line dot.

It seems to me that generate_dot() should use double quotes by default since that's what dot expects.

[olivroy]

Other double quotes / single quotes issues #433, #432, #441