Add timeline plot with plotly as backend

[eukaryo]

Motivation

To provide an easy way to check the execution status, I want to implement a function to plot the Trials as a timeline.
The timeline here is a Gantt chart with time on the horizontal axis and trial number on the vertical axis, which makes it clear at a glance whether trials are being executed in parallel and whether there are any trials with abnormally long execution times.

Description of the changes

• Add timeline plot using plotly's horizontal bar graph in optuna/visualization/_timeline.py.

• Add Unit tests in tests/visualization_tests/test_timeline.py.

[toshihikoyanase]

Thank you for your PR! Let me check it.
If possible, could you add a figure of a example plot to the description, please?

[eukaryo]

I see. I have added a figure of timeline plot to the description above. Here is sample code to generate the figure, which is basically the same code as in the docstring of optuna.visualization.plot_timeline.

import time
import plotly
import optuna

def objective(trial):
    x = trial.suggest_float("x", 0, 1)
    time.sleep(x * 0.1)
    if x > 0.8:
        raise ValueError()
    if x > 0.4:
        raise optuna.TrialPruned()
    return x**2

study = optuna.create_study(direction="minimize")
study.enqueue_trial({"x": 0.3})
study.enqueue_trial({"x": 0.5})
study.enqueue_trial({"x": 0.9})
study.optimize(objective, n_trials=50, n_jobs=2, catch=(ValueError,))

ax = optuna.visualization.plot_timeline(study)
plotly.io.write_image(ax, "test-fig-plotly.png", format="png")

[HideakiImamura]

Thanks for the PR. I have several comments. Please take a look.

[codecov-commenter]

Codecov Report

Merging #4470 (19750bd) into master (65558b7) will increase coverage by 0.02%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #4470      +/-   ##
==========================================
+ Coverage   90.32%   90.35%   +0.02%     
==========================================
  Files         183      184       +1     
  Lines       14142    14191      +49     
==========================================
+ Hits        12774    12822      +48     
- Misses       1368     1369       +1     

Impacted Files	Coverage Δ
optuna/visualization/__init__.py	100.00% <100.00%> (ø)
optuna/visualization/_pareto_front.py	97.08% <100.00%> (-0.22%)	⬇️
optuna/visualization/_timeline.py	100.00% <100.00%> (ø)
optuna/visualization/_utils.py	95.55% <100.00%> (+0.68%)	⬆️

... and 1 file with indirect coverage changes

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[toshihikoyanase]

Thank you for your PR and example. Let me share some comments on the implementation of the timeline plot.
Next, I'll check the test cases.

[toshihikoyanase]

Let me share my comments on the tests.

[toshihikoyanase]

Thank you for your update. Overall, it looks almost good to me, but I have some additional comments.

[HideakiImamura]

Thanks for the update. Basically, LGTM. I have 2 minor comments. PTAL.

[toshihikoyanase]

Thank you for your update. LGTM!

[HideakiImamura]

Thanks for the update! LGTM!