Oops! I misread it! it does support it, and exactly by the code I presented above. Ok, so that IS officially supported.

When I run into these things I always look at the examples, it generally gives a good feel about what the various inputs/outputs are.

Good to hear you got it resolved!

To add:

Note that in the TreePlot documentation a Graph object is not a valid input. There is however some 'hidden' (undocumented) functionality inside the definition of Treeplot. Namely:

TreePlot[g_ ? GraphQ, expr___] := Block[{res, adj, opts},
    adj = AdjacencyMatrix @ g;
    Condition[
       opts = opts = toTreePlotOptions @ g;
       res = TreePlot[adj, expr, opts];
       res /; UnsameQ[res, $Failed],
       MatrixQ[adj]
    ]
]

You can see how that it can handle Graph objects in some ways, but note that this is undocumented and might as well get removed in future versions! It might be some remnants from development (opts = opts = makes me think this).

Standard Graph- or TreeGraph-objects or their defining data are obviously useless. It's a horrible mess.

I want to solve some problem, but not debug WL.

I am really stuck. Bugs or not within all those WL-functions defining and exploring or plotting graphs, I cannot understand them. Maybe my test-program got too complicated. I reduced it to almost nothing.

What is wrong with the following trivial code? (Sorry I could not find how to show code and its output in a reasonable way in this forum).

{graphDataVertex, graphDataEdge} = {
  {Labeled[a, "S=a"], b1, b2, Labeled[c, "E=c"], Labeled[c2, "E=c2"]},
  {Labeled[a \[DirectedEdge] b1, "a,b1"], 
   Labeled[a \[DirectedEdge] b2, "a,b2"], b1 \[DirectedEdge] c, 
   b2 \[DirectedEdge] c2}
  }
graph = Graph[graphDataVertex, graphDataEdge, ImageSize -> Tiny]
FindPath[graph, a, c, Infinity, All]
TreePlot[graph, Left, a]
treeGraph = 
 TreeGraph[graphDataVertex, graphDataEdge, ImageSize -> Tiny]
FindPath[treeGraph, a, c2, Infinity, All]
TreePlot[treeGraph, Left, a]

Strange enough the following TreePlot-calls without startnode and its position work. But why?

TreePlot[graph]
TreePlot[treeGraph]

@ Werner: "->" is not the same as \DirectedEdge, otherwise I would have no reason to changed it. Did you run the code with my corrections in it (as posted above)?

Hello Werner,

replace

graphData = {Labeled[a -> b1, "a,b1"], Labeled[a -> b2, "a,b2"], 
   b1 -> c, b2 -> c};

by

graphData = {Labeled[a \[DirectedEdge] b1, "a,b1"], 
   Labeled[a \[DirectedEdge] b2, "a,b2"], b1 \[DirectedEdge] c, 
   b2 \[DirectedEdge] c};

Treeplot requires a graph not just graphdata:

Print[TreePlot[Graph[graphData], 
   PlotLabel -> Style["1.1 TreePlot from graphData", Blue, Bold], 
   DirectedEdges -> True, VertexLabeling -> True, 
   EdgeLabeling -> True]];

Note that I also took out the "a". With these changes the 1st part should run.

I copied the changed version in below:

If[True,(*Test Graph (from Graph-Data)& TreePlot& FindPath*)
 Block[{a, b1, b2, c, graphData, 
   graph},(*Define graphData as a labeled list of edges in a form \
suitable for Graph*)
  Print[Style["1 Defining a graph by graphData suitable for Graph", 
    Red, Bold]];
  (* graphData={Labeled[a\[Rule]b1,"a,b1"],Labeled[a\[Rule]b2,"a,b2"],
  b1\[Rule]c,b2\[Rule]c};*)
  graphData = {Labeled[a \[DirectedEdge] b1, "a,b1"], 
    Labeled[a \[DirectedEdge] b2, "a,b2"], b1 \[DirectedEdge] c, 
    b2 \[DirectedEdge] c};
  Echo[graphData, "1 graphData: "];
  (*Make a Graph-object from graphData*)graph = Graph[graphData];
  Echo[Row[{Head[graph], " ", graph}], 
   "1 make graph from graphData: "];
  (*TreePlot and FindPath from graphData*)(* Print[TreePlot[graphData,
  Left,a,PlotLabel\[Rule]Style["1.1 TreePlot from graphData",Blue,
  Bold],DirectedEdges\[Rule]True,VertexLabeling\[Rule]True,
  EdgeLabeling\[Rule]True]];*)
  Print[TreePlot[Graph[graphData], 
    PlotLabel -> Style["1.1 TreePlot from graphData", Blue, Bold], 
    DirectedEdges -> True, VertexLabeling -> True, 
    EdgeLabeling -> True]];
  (* Print[Style[
  Row[{"1.1 FindPath from graphData: All paths from a to c: ",
  FindPath[graphData,a,c,Infinity,All]}],Blue,Bold]];*)
  Print[Style[
    Row[{"1.1 FindPath from graphData: All paths from a to c: ", 
      FindPath[Graph[graphData], a, c, Infinity, All]}], Blue, Bold]];
  (*TreePlot and FindPath from graph*)
  Print[TreePlot[graph, Left, 
    PlotLabel -> Style["1.2 TreePlot from graph", Blue, Bold], 
    DirectedEdges -> True, VertexLabeling -> True, 
    EdgeLabeling -> True]];
  Print[Style[
    Row[{"1.2 FindPath from graph: All paths from a to c: ", 
      FindPath[graph, a, c, Infinity, All]}], Blue, Bold]];
  (*Define graphData as a labeled list of edges in a Form suitable \
for TreePlot*)
  Print[Style["2 Defining a graph by graphData suitable for TreePlot",
     Red, Bold]];
  (* graphData={{a\[Rule]b1,"a,b1"},{a\[Rule]b2,"a,b2"},b1\[Rule]c,
  b2\[Rule]c};*)
  graphData = {Labeled[a \[DirectedEdge] b1, "a,b1"], 
    Labeled[a \[DirectedEdge] b2, "a,b2"], b1 \[DirectedEdge] c, 
    b2 \[DirectedEdge] c};
  Echo[graphData, "2 graphData: "];
  (*Make a Graph-object from graphData*)graph = Graph[graphData];
  Echo[Row[{Head[graph], " ", graph}], 
   "2 make graph from graphData: "];
  (*TreePlot and FindPath from graphData*)(* Print[TreePlot[graphData,
  Left,a,PlotLabel\[Rule]Style["2.1 TreePlot from graphData",Blue,
  Bold],DirectedEdges\[Rule]True,VertexLabeling\[Rule]True,
  EdgeLabeling\[Rule]True]];*)
  Print[TreePlot[Graph[graphData], 
    PlotLabel -> Style["2.1 TreePlot from graphData", Blue, Bold], 
    DirectedEdges -> True, VertexLabeling -> True, 
    EdgeLabeling -> True]];
  Print[Style[
    Row[{"2.1 FindPath from graphData: All paths from a to c: ", 
      FindPath[Graph[graphData], a, c, Infinity, All]}], Blue, Bold]];
  (*TreePlot and FindPath from graph*)
  Print[TreePlot[graph, Left, 
    PlotLabel -> Style["2.2 TreePlot from graph", Blue, Bold], 
    DirectedEdges -> True, VertexLabeling -> True, 
    EdgeLabeling -> True]];
  Print[Style[
    Row[{"2.2 FindPath from graph: All paths from a to c: ", 
      FindPath[graph, a, c, Infinity, All]}], Blue, Bold]];]]