Tooltip content

[lmacko]

Pull request includes following changes:

• precision in x or y value Problem with precision in the x or y value #50
  • concerning $ escaping tooltip displays without leading $1 #16
• use custom ticks if given Tooltip does not replace x label by given x. #18
• remove %s if series label is undefined %s stays in tooltip if there is no series label #41

I've also added two examples showing $ escaping and custom ticks usage.

[krzysu]

looks like great job, many thanks!