Add fonctionnality groupBySeries and groupById

[GuillaumeSmaha]

Hey,
To keep my master equals to upstream, I created a new branch for this feature and I clean it to minimize modification (indent, ...)
So, I will close PR #26
Feature:

• groupBySeries permit to aggregate all elements in the serie to the same row
• groupById permit to aggregate all rows with the same id to an uniq row.
• Add opacity for not hover element"
• Add option groupByIdDrawAllTitles: This option permit to draw all titles
• when the option groupById is enabled else draw only the first title
• When groupByIdDrawAllTitles is enabled, cellHeight is auto sized

[GuillaumeSmaha]

@lutze Even if you thought to do it in a different way, I prefer to keep the PR alive until your implementation