To do #786
User story #783: As a user I want to see values in the burndown graphics when a mouse over the points
Change Sprint burndown chart to render info on mouse over the values
- Gruff library doesn't support the mouse over because it renders a PNG image.
- Search new library to render charts.
- Integrate it.
- Refactor the Sprint burndown chart to use it (including values tooltips on mouse over).