New feature in Calc: Stepped Lines graphs

Hello everyone,

in order to scratch my own itch, I started working on a stepped lines feature 
for XY graphs (and Line graphs) in Calc. See bugzilla 61135. This is my first 
coding in LO :-)

Next steps would be to expose this functionality in the UI and check that it 
would be saved and read in the file format.

However, before spending more time, I would like to know about the following:
- can we just add a new type of graph ? Don't we have to follow the ODF 1.2 
specification ?
- if I add this, what about compatibility with older LO/OOo versions and other 
spreadsheets using ODF (Gnumeric, KOffice, ...) ?
- how to handle UI changes ? Can I create a patch/proposal independently ?

Thanks for the answers,

Eric