Switch the default driver for export_png to chromedriver

Started at #5714. All relevant details are repeated here.

PhantomJS is no longer maintained, and at this point using it produces PNGs like these: (a fragment with y-axis) (a fragment with x-axis with rotated labels) Using normal font instead of the italic one does not mitigate the issue.

Currently, I’m using this code to create a driver instance and pass it down to export_png:

import os
import shutil

from chromedriver_binary.utils import get_chromedriver_path
from selenium.webdriver import Chrome, ChromeOptions

options = ChromeOptions()
# TODO: Remove when https://bugs.chromium.org/p/chromium/issues/detail?id=875241 is fixed
options.binary_location = shutil.which('chrome')

options.add_argument('--headless')
options.add_argument('--disable-gpu')
options.add_argument("--no-sandbox")
options.add_argument("--window-size=2000x2000")

web_driver = Chrome(executable_path=os.path.join(get_chromedriver_path(), 'chromedriver'),
                    chrome_options=options)

chromedriver_binary is available by installing python-chromedriver-binary from the conda-forge channel. I tested my code with version 2.41.0.

There’s a few of issues:

1. Google Chrome can be available under names other than chrome
2. There’s no check for Google Chrome version
3. The fixed window size, 2000x2000. It probably could be somehow computed in advance
4. Need to check whether this could be used with Chromium - many people use it instead of Google Chrome

Also, maybe at some point there could be a simple option that would use Firefox, but right now it appears to be unusable as it complains about missing log command.

[bryevdv]

Here is a complete example that sets the device pixel ratio to 1.0 to get a correctly scaled image on OSX:

from selenium.webdriver import Chrome, ChromeOptions

options = ChromeOptions()

options.add_argument('--headless')
options.add_argument("--window-size=2000x2000")
metrics = { "deviceMetrics": { "pixelRatio": 1.0 } }
options.add_experimental_option("mobileEmulation", metrics)
web_driver = Chrome(chrome_options=options)

import numpy as np

from bokeh.plotting import figure, show, output_file

N = 4000
x = np.random.random(size=N) * 100
y = np.random.random(size=N) * 100
radii = np.random.random(size=N) * 1.5
colors = [
    "#%02x%02x%02x" % (int(r), int(g), 150) for r, g in zip(50+2*x, 30+2*y)
]

p = figure(tools="")

p.circle(x, y, radius=radii,
          fill_color=colors, fill_alpha=0.6,
          line_color=None)

from bokeh.io import export_png

export_png(p, "out.png", webdriver=web_driver)

It’s unclear to me yet if just unconditionally setting to 1.0 is always the correct thing to do, or if it is a specific action for a macbook w retina display.

[mattpap]

This was resolved in PR #9331.