How to position the legend at the bottom-left of the graph in a Graphviz flowchart?

I'm working on a flowchart using Graphviz and need to position the legend at the bottom-left of the graph. Below is the code I am currently using, where I have added a subgraph for the legend. By default, the legend is placed at the top, but I would like to move it to the bottom-left.

Here’s a simplified version of my code:

digraph flowchart {
  
  # Graph
  graph [
    overlap = true
    fontsize = 10
  ]
  
  # Define styles for Datasets and Analyses
  node [fontname = 'Calibri Light', fontcolor = white, style = filled]
  
  # Datasets (boxes)
  node [shape = box, fillcolor = firebrick3]  # Set dataset style
  A
  B
  
  # Analyses (circles)
  node [shape = circle, fixedsize = true, width = .9, fillcolor = steelblue3]
  C
  D
  
  # Edges
  {A B}->C
  A->D
  
  # Subgraph for legend
  subgraph cluster_legend {
    graph [
      label = 'Legend'
      labelloc = 't'
      labeljust= 'l'
      fontname = 'Calibri Light'
      color = lightgrey
    ]
    
    # Global node style
    node [fontsize = 9]
    
    # Legend entry for Analysis
    node [shape = circle, fixedsize = true, fillcolor = steelblue3, width = .5, height = .3]
    legend_analysis [label = 'Analysis']
    
    # Legend entry for Data
    node [shape = box, fillcolor = firebrick3, width = .5, height = .5]
    legend_data [label = 'Data']
    
    # Make legend items horizontal
    {rank = same; legend_data; legend_analysis}
  }
}

Could anyone suggest how I can move the legend to the bottom-left of the graph?

Solution

Here is one way.

Split the graph into two complete graphs (same input file) (see below)
run through dot, producing xdot output format (https://www.graphviz.org/docs/outputs/canon/)
run that through gvpack (https://www.graphviz.org/pdf/gvpack.1.pdf) to combine the two graphs, with arguments -array_il1 to stack the two graphs one-over-the-other, left justified
run that through neato -n2 -T... to produce the image. (see https://www.graphviz.org/faq/#FaqDotWithCoords)

Kind of convoluted, but this way you don't need invisible nodes/edges to try to coerce the Legend into the desired position.

/*******************************************************************
  create a second graph containing the legend
  use gvpack to combine the two,
  then neato -n2 to create image
********************************************************************/
digraph flowchart {
  subgraph cluster_main {  //  sometimes helps w/ gvpack
  # Graph
  graph [
    //  overlap = true  //  dot ignores overlap
    fontsize = 10
  ]
  
  # Define styles for Datasets and Analyses
  node [fontname = "Calibri Light", fontcolor = white, style = filled]
  
  # Datasets (boxes)
  node [shape = box, fillcolor = firebrick3]  # Set dataset style
  A
  B
  
  # Analyses (circles)
  node [shape = circle, fixedsize = true, width = .9, fillcolor = steelblue3]
  C
  D
  
  # Edges
  {A B}->C
  A->D
  }
}
/**  now the legend  **/
digraph L {
 # Subgraph for legend
  subgraph cluster_legend {
    graph [
      label = "Legend"
      labelloc = "t"
      labeljust= "l"
      fontname = "Calibri Light"
      color = lightgrey
    ]
    
    # Global node style
    node [fontsize = 9]
    
    # Legend entry for Analysis
    node [shape = circle, fixedsize = true, fillcolor = steelblue3, width = .5, height = .3]
    legend_analysis [label = "Analysis"]
    
    # Legend entry for Data
    node [shape = box, fillcolor = firebrick3, width = .5, height = .5]
    legend_data [label = "Data"]
    
    # Make legend items horizontal
    {rank = same; legend_data; legend_analysis}
  }
}

And command line (note dot output format is xdot):

dot myFile.gv | gvpack -array_il1 |neato -n2 -T...

Giving: