replace default tooltips 'n/a'

[4-1-1]

Enable the ability to Change the init display, not only 'n/a'

[rrag]

Thank you. can you give me an example where x and y initial display will be different?

Being consistent with OHLCTooltip would be my choice

adding a na prop and using it like here
https://github.com/rrag/react-stockcharts/blob/master/src/lib/tooltip/OHLCTooltip.js#L103

[4-1-1]

@rrag Yes, you are right, one props should be enough
@benmccann I'm sorry to do not run the code before commit, It's my mistake