How can I achieve a multiline title for a holoviews plot?

Question

I would like more than one line as an option for a title of a holoviews plot object. Only strings are allowed, and it seems what works for a print statement does not work for a title.

import numpy as np
import holoviews as hv
hv.extension('bokeh')
from holoviews import opts

plot_title = 'Line 1 \nLine 2'
plot_title
# printing results in two lines, the \n is recognized
print(plot_title)

# the \n is ignored when the string is used for a title
points = [(0.1*i, np.sin(0.1*i)) for i in range(100)]
hv.Curve(points).opts(title=plot_title)

Answer 1

HoloViews uses bokeh as a plotting backend in your example and this feature, i.e multi-line title, isn't supported yet by bokeh. Note that your question is similar to this one: How to create a multi-line plot title in bokeh? and that there is now (27/04/2020) a related open issue on bokeh https://github.com/bokeh/bokeh/issues/7317.

I see two ways of getting multilines title for your plot.

The first one is adapted from the other SO question and makes use of bokeh directly to add titles as a layout:

import numpy as np
import holoviews as hv
import bokeh.io
from bokeh.models import Title
hv.extension('bokeh')

points = [(0.1*i, np.sin(0.1*i)) for i in range(100)]

hv_curve = hv.Curve(points)
bk_curve = hv.render(hv_curve)
bk_curve.add_layout(Title(text="Sub-Title", text_font_style="italic"), 'above')
bk_curve.add_layout(Title(text="Title", text_font_size="16pt", text_font_style="bold"), 'above')
bokeh.io.show(bk_curve)

The second one makes use of panel (a dependency of HoloViews now) to display the titles as two Markdown panes (it could also be one HTML pane) both centered in a Column layout above the curve:

import numpy as np
import holoviews as hv
hv.extension('bokeh')
import panel as pn
pn.extension()

points = [(0.1*i, np.sin(0.1*i)) for i in range(100)]

hv_curve = hv.Curve(points)
panel_layout pn.Column(
    pn.pane.Markdown("**Title**", align="center"),  # bold
    pn.pane.Markdown("*Sub-title*", align="center"),  # italic
    hv_curve
)
panel_layout

If you run panel_layout.pprint() you'll be able to check the structure of that layout.

Column
    [0] Markdown(str, align='center')
    [1] Markdown(str, align='center')
    [2] HoloViews(Curve)

Note: Run with Holoviews 1.13.2, bokeh 2.0.1 and panel 0.9.5.