Hi Olivier, please take a look at How to split lines based on a term in Timelion using split() or other? for more information on using split in Timelion. Converting values to lowercase isn't part of Timelion, though. To do that part, you'll need to create a scripted field under Management > Index Patterns > Scripted Fields. You'll need to configure it as type "string" with a script like this:
doc['name_of_your_field_here'].value.toLowerCase()
Then set your Timelion graph to split using this scripted field.
Hope this helps,
CJ