Use Vega-Lite layering to support tooltip on line charts and area charts

[light-and-salt]

Currently tooltip for line chart does not report correct data because data is bound to path, not to points. In the screenshot below, price should be a number around 200 and date should be in human-readable format (that's another issue).

[kanitw]

This will be partially addressed by fixing vega/vega-lite#508, but we should wait until we have layering support first.

[kanitw]

One thing that we can do now is

• from the item parameter we get. Is there a way to know that the mark is a line, then we can drop quantitative values?

[light-and-salt]

Partially fixed in PR #16, which only displays qualitative fields for line charts and area charts, and gives warnings in the console.

[kanitw]

Hmm. If it is partially fixed and should be further fixed, I don't think we should close it.

[light-and-salt]

Dropping quantitative fields does seem to solve the problem for the line_color example we have above, but if the stocks.csv data set contains some additional qualitative fields, shouldn't those fields be dropped too? An additional field's value will vary for a line/area mark, but before we add layering support, tooltip will only pick up one datum for a line/area mark, which may be inaccurate.

As a better partial fix, we may drop fields in the x and y channels and only show fields in the other encoding channels (typically color). In this way, if a qualitative field is not used for encoding, it will not show up in the tooltip.

I am writing down this thought for logging purpose. I know we will have layering really soon, so we may not even need to implement this "better partial fix" -- we may go straight into "using layering to properly show all fields" for line charts and area charts.

I wrote down these thoughts as comments in PR #16.

[light-and-salt]

Close via #50