I'm struggling to get 2 traces on a plotly graph when updating the data from a javascript clientside callback.

I have this clientside callback:

app.clientside_callback(
    ClientsideFunction(
        namespace='clientside',
        function_name='update_rms_graph'
    ),
    Output('rmstrace-graph', 'extendData'),
    (
        Input('client-rms-data', 'data'),
        Input('client-rms-2-data', 'data'),
        Input('timer-refresh-rms-fft', 'n_intervals')
    )
)

Which is defined as such:

window.dash_clientside = Object.assign({}, window.dash_clientside, {
    clientside: {
            update_rms_graph: function(data, data_rms, n_intervals) {
                console.log(data.rms.length, data_rms.rms.length);
                return [
                    {y: [data.rms],  x: [data.t]},
                    //{y: [data_rms.rms],  x: [data_rms.t]}, // This is where i'd like to add my new trace
                    [0],
                    data.max_points
                ]
            }
    }
)

The 'rmstrace-graph' is defined as such:

html.Div([
    dcc.Graph(id='rmstrace-graph', figure=fig_rmstrace),
], className='six columns')

And the corresponding figure used for the dash graph is the following:

fig_rmstrace = go.Figure(data=go.Scattergl(y=[], x=[]))

How should the 'return' from the clientside callback be wrote if I like to get a second trace from the same dash graph ?