Plotly legend title

I've done this before by making a data-less trace

import plotly.plotly as py
import plotly.graph_objs as go

dummy_trace = go.Scatter(
    x=[None], y=[None],
    name='<b>Legend Heading</b>',
    # set opacity = 0
    line={'color': 'rgba(0, 0, 0, 0)'}
)

trace0 = go.Scatter(
    x=[1, 2, 3, 4, 5],
    y=[1, 2, 3, 4, 5],
)

trace1 = go.Scatter(
    x=[1, 2, 3, 4, 5],
    y=[5, 4, 3, 2, 1],
)

data = [dummy_trace, trace0, trace1]
fig = go.Figure(data=data)

py.iplot(fig)

Update:

For not defining the legend but having the annotation positioned property please use the below code.

import plotly.offline as py_offline
import plotly.graph_objs as go
py_offline.init_notebook_mode()

trace0 = go.Scatter(
    x=[1, 2, 3, 4, 5],
    y=[1, 2, 3, 4, 5],
)

trace1 = go.Scatter(
    x=[1, 2, 3, 4, 5],
    y=[5, 4, 3, 2, 1],
)

data = [trace0, trace1]
layout = go.Layout(
    annotations=[
        dict(
            x=1.12,
            y=1.05,
            align="right",
            valign="top",
            text='Legend Title',
            showarrow=False,
            xref="paper",
            yref="paper",
            xanchor="center",
            yanchor="top"
        )
    ]
)
fig = go.Figure(data=data, layout = layout)

py_offline.iplot(fig)

Notes:

1. You need to define x and y position for annotations using this method, for varying legends.

2. You can use html inside the text attribute(E.g: text='Legend Title<br>kinda lengthy',)

Previous Attempt:

Another approach would to create the legend and use annotations to add the title to the legend. Provided you do not use the graph in editable mode. So in the below example, the legend is set to x=0 and y=1, since I want my legend title to be above my actual legend, I set the annotation location as x = 0, y= 1.5. x-ref and y-ref needs to be set to paper. This will give a nice annotation like

Code:

import plotly.plotly as py
import plotly.graph_objs as go

trace0 = go.Scatter(
    x=[1, 2, 3, 4, 5],
    y=[1, 2, 3, 4, 5],
)

trace1 = go.Scatter(
    x=[1, 2, 3, 4, 5],
    y=[5, 4, 3, 2, 1],
)

data = [trace0, trace1]
layout = go.Layout(
    legend=dict(
        x=0,
        y=1,
        traceorder='normal',
        font=dict(
            family='sans-serif',
            size=12,
            color='#000'
        ),
        bgcolor='#E2E2E2',
        bordercolor='#FFFFFF',
        borderwidth=2
    ),
    annotations=[
        dict(
            x=0,
            y=1.05,
            xref='paper',
            yref='paper',
            text='Legend Title',
            showarrow=False
        )
    ]
)
fig = go.Figure(data=data, layout = layout)

py.iplot(fig)