Thanks for contributing this.
I installed your filter on a fresh Moodle install and was able to enable it without problems.
I copied the example XML file to the files area of a test course. I went to insert an example of a timeline to a Web page resource. I copied the XML example filter syntax provided, removing leading space, into the WYSIWYG editor, and replaced the course id. When I saved the page and displayed it, it reported an error...
Error, missing 'date' parameter.
The date parameter was in the filter entry. I wondered if perhaps there was too much whitespace around the name and value and the = symbol. I removed the space for the date parameter and tried to view the page again. This time the error did not appear but other errors did, including...
Notice: Undefined property: stdClass::$intervalUnit in D:\xampplite\htdocs\moodle\filter\timelinewidget\filter.php on line 172
...so I removed all whitespace around the = symbols for parameters and saved again. Is there some specific requirement or limit for space? This time, the timeline appeared, but there was an error trace displayed...
line 119 of filter\timelinewidget\filter.php: call to debugging()
line ? of unknownfile: call to _timeline_filter_callback()
line 71 of filter\timelinewidget\filter.php: call to preg_replace_callback()
line 1838 of lib\weblib.php: call to timelinewidget_filter()
line 1588 of lib\weblib.php: call to filter_text()
line 120 of mod\resource\type\html\resource.class.php: call to format_text()
line 40 of mod\resource\view.php: call to resource_html->display()
Interestingly, when I refreshed, these errors seemed to go away, but the returned when I refreshed a second time. When I clicked on an event in the timeline, a bubble appeared showing information about the event. When the bubbles first appeared they had vertical and horizontal scrollbars that squashed the content a bit. When I viewed the content a second time, these scrollbars were not present and the event appeared nicely. I'll include a screen capture showing the error trace and scrollbars.
I would like to make some recommendations for the filter syntax. Can I suggest that most of the parameters be made optional. For example, the title could be left out, the inital viewing date could be based on the dates in the timeline, the interval could be a default or calculated, the intervalPixels could be a default or calculated. The only necessary parameters seem to be related to the source of the timeline information. In my experience, while it's nice to have control (and this could remain if the user does provide these parameters), most teachers need plugin configuration to be as simple as possible.
I also wanted to ask where you want to go with this contribution, once you have a version of the filter you are happy with. You have a repository for your code presently. You can serve it from there, but most contributed plugins are accessed by downloading a zip created from the Moodle CVS repository. This has many advantages and I recommend it. Either way you can have an entry for bugs in the Tracker and an entry in the Modules and Plugins database.
Let me know how you want to progress from here. What you're contributing will be valuable to many people, so I'm happy to continue testing and supporting this to go further.