how to set the bold font style in Plotly

If you are using Python you can add the <b> html tag to the title attribute and you are good.

You can add some more limited HTML styling such as italic as well, see example below.

import plotly
plotly.offline.init_notebook_mode()
data = [plotly.graph_objs.Bar(
            x=['giraffes', 'orangutans', 'monkeys'],
            y=[20, 14, 23]
    )]
layout = go.Layout(title="<b>Bold</b> <i>animals</i>")

fig = plotly.graph_objs.Figure(data=data, layout=layout)
plotly.offline.iplot(fig)