Bokeh - how to add y value to y-axis of horizontal Span?

Question

How can I show and highlight the value of the horizontal Span on the y-axis?

import bokeh as bk
import bokeh.plotting as bkplot
#bkplot.output_notebook() # to show inline

x = np.arange(3)
y = x**2

source = bk.models.ColumnDataSource(dict(x=x, y=y))

p = bkplot.figure(plot_width=500, plot_height=300)

glyph = bk.models.Line(x="x", y="y", line_width=6)
p.add_glyph(source, glyph)

top_span = bk.models.Span(location=3.5, dimension='width', line_color='green', line_width=2)
bottom_span = bk.models.Span(location=1.5, dimension='width', line_color='red', line_width=2)
p.renderers.extend([top_span, bottom_span])

bkplot.show(p)

Result:

Desired result:

Answer 1

There's been a discussion of this very issue in Bokeh's github: https://github.com/bokeh/bokeh/issues/7309

So having labels on Spans in the axis area vs. the plot area is not yet implemented.

If you're okay with labels in the plot area, though, you may want to use a Label. Here's what I was able to do with Labels:

top_span = bk.models.Span(location=3.5, dimension='width', line_color='green', line_width=2)
top_span_label = bk.models.Label(text_color='green', text=str(top_span.location), x=0, y=top_span.location)
bottom_span = bk.models.Span(location=1.5, dimension='width', line_color='red', line_width=2)
bottom_span_label = bk.models.Label(text_color=bottom_span.line_color, text=str(bottom_span.location), x=0, y=bottom_span.location)
p.renderers.extend([top_span, top_span_label, bottom_span, bottom_span_label])