Plotly Tree Graph Not Creating The Root Node

I'm not an igraph specialist, so my suggestion may have some flaws, but it will enable you to set 'david' as the root node by simply changing the line:

lay = G.layout_reingold_tilford(mode="in", root=0)

to:

lay = G.layout_reingold_tilford(mode="in", root=1)

This seems to be because G is constructed using:

G=Graph()

#G.add_vertices(l)
G.add_vertices(vertices)

# add edges to the graph
G.add_edges(l)

And vertices is:

['claire', 'david', 'jenni', 'john', 'mavri']

And 0 in lay = G.layout_reingold_tilford(mode="in", root=0) sets 'claire' as the root node. So it turns out that you can set thee root to any element in ['claire', 'david', 'jenni', 'john', 'mavri'] by specifying the corresponding index. So lay = G.layout_reingold_tilford(mode="in", root=1) produces the plot below. And you can verify my findings by changing 1 to something else.

Complete code:

import igraph
from igraph import Graph, EdgeSeq
nr_vertices = 25

import plotly.offline as pyo
pyo.init_notebook_mode()


l=[('david','john'),
   ('david','jenni'), 
    ('john','jenni'),
   ('john','david'),
   ('john','mavri'),
   ('john','claire'),
   ]

    
vertices = set()
for line in l:
    vertices.update(line)
vertices = sorted(vertices)

print(len(l))

v_label = list(map(str, range(len(vertices))))
v2 = list(map(str, vertices))
#g = Graph.TupleList(directed=True, edges = l) 
G=Graph()

#G.add_vertices(l)
G.add_vertices(vertices)

# add edges to the graph
G.add_edges(l)


lay = G.layout_reingold_tilford(mode="in", root=1)
print(G)

position = {k: lay[k] for k in range(len(vertices))}

print(position)

Y = [lay[k][1] for k in range(len(vertices))]

M = max(Y)


es = EdgeSeq(G) # sequence of edges
E = [e.tuple for e in G.es] # list of edges
print(E)

L = len(position)

Xn = [position[k][0] for k in range(L)]

Yn = [2*M-position[k][1] for k in range(L)]
Xe = []
Ye = []

for edge in E:
    Xe+=[position[edge[0]][0],position[edge[1]][0], None]
    Ye+=[2*M-position[edge[0]][1],2*M-position[edge[1]][1], None]

labels = v2
print(Xe)

import plotly.graph_objects as go
fig = go.Figure()
fig.add_trace(go.Scatter(x=Xe,
                   y=Ye,
                   mode='lines',
                   line=dict(color='rgb(210,210,210)', width=1),
                   hoverinfo='none'
                   ))
fig.add_trace(go.Scatter(x=Xn,
                  y=Yn,
                  mode='markers',
                  name='bla',
                  marker=dict(symbol='circle-dot',
                                size=30,
                                color='#6175c1',    #'#DB4551',
                                line=dict(color='rgb(50,50,50)', width=1)
                                ),
                  text=labels,
                  hoverinfo='text',
                  opacity=0.8
                  ))

def make_annotations(pos, text, font_size=10, font_color='rgb(250,250,250)'):
    L=len(pos)
    if len(text)!=L:
        raise ValueError('The lists pos and text must have the same len')
    annotations = []
    for k in range(L):
        annotations.append(
            dict(
                text=labels[k], # or replace labels with a different list for the text within the circle
                x=pos[k][0], y=2*M-position[k][1],
                xref='x1', yref='y1',
                font=dict(color=font_color, size=font_size),
                showarrow=False)
        )
    return annotations

axis = dict(showline=False, # hide axis line, grid, ticklabels and  title
            zeroline=False,
            showgrid=False,
            showticklabels=False,
            )

fig.update_layout(title= 'Tree with Reingold-Tilford Layout',
              annotations=make_annotations(position, v_label),
              font_size=12,
              showlegend=False,
              xaxis=axis,
              yaxis=axis,
              margin=dict(l=40, r=40, b=85, t=100),
              hovermode='closest',
              plot_bgcolor='rgb(248,248,248)'
              )
fig.show()