Provide the subtitle option

I'm a little bit expecting this to not apply because I am missing a previous small commit, but checking just in case.

OK, trying again.

The last submitted patch, 11: 3293577-subtitle-11.patch, failed testing. View results →
- codesniffer_fixes.patch Interdiff of automated coding standards fixes only.

This should fix the broken test and a few of the new code style issues.

Happy to see this pass!

Some notes:

1. Highcharts and Chart.js have full support for a subtitle option (yay!)
2. Billboard.js supports a MultilineTitle, so I used this and placed the subtitle into the second line. Needed to update the library versions for this to work well.
3. C3.js does not support a subtitle or multi-line title. Because the subtitle field is going to show in all the settings forms, I want it to have an effect if a user adds a value. What I do is just append the subtitle to the title with ": " as a separator. If users don't want this separator, they can just use the title field and leave the subtitle field blank.
4. Google Charts is pretty complicated. The "corechart" package does not support subtitles, so where "corechart" is used, I apply the same solution as described above for C3.js (trying to be consistent as possible). However, for a handful of chart types (Bar/Column, Line/Spline, and Scatter), Google has a "Material Design" option. The Material Design option does support a subtitle. To make use of Material Design charts, navigate to /admin/config/content/charts, set Google as your default library, click the "Google settings" tab, and then check the "Use Material Design" checkbox. Then, for any of the above-mentioned chart types, you will see your chart rendered using the Material Design package and it will show a subtitle as a true subtitle rather than just the title with a subtitle appended. You can read a little more about the Material Design implementation here: https://developers.google.com/chart/interactive/docs/gallery/barchart#cr....

FYI: I plan to implement the subtitle feature in the contrib charts_echarts module as soon as this patch goes through (it also has true support for a subtitle).

One more patch (hoping to be the last) that adds a couple features:

1. Uses the "states" property to disable the subtitle until a value is added for the title.
2. Allows use of tokens in views.

Couple other small tweaks.

Automatically closed - issue fixed for 2 weeks with no activity.

The new field "subtitle" leads to a warning for existing content: Warning: Undefined array key "subtitle" in Drupal\charts\Element\Chart::buildElement() (line 322 of modules/contrib/charts/src/Element/Chart.php).

If you save the corresponding content again, the warning disappears.

I suspect that the field "subtitle" is not stored in the database and therefore is fixed when saving again.

But this is not reasonable for some websites, 30k+ content.

Any suggestions for a solution?