add rollover formatting options

[hamilton]

The default rollover formatting options are very limited. We would like to include custom formatting options that are both powerful and easy. We will be including two new params: x_rollover_format and y_rollover_format. It takes either a string (depending on what the data type is), or a function (for other / extended cases).

STRING

When the accessor is a number, we will provide D3's native number formatting options: https://github.com/mbostock/d3/wiki/Formatting

When the acessor is a date, we will provide D3's native date formatting options: https://github.com/mbostock/d3/wiki/Time-Formatting

FUNCTION

If you pass in a function, you get the data point, as before. This should allow for every other useful case we care about.

[dgilperez]

+1

[hamilton]

@almossawi one way we can make this easier to format is to make a series of tspans within the rollover text element, each one corresponding to the x accessor, y accessor, and perhaps something relating to the group if we have a multiline element. This should then reduce the problem to merely formatting tspans and changing the text within them, which should open up possibilities for other formatting options, if we want to go that direction.

[almossawi]

@hamilton Yes, as discussed. That sounds great.

[hamilton]

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