implement disp.y in pathBuilders

currently stacked [1] and shifted [2] demos have to pre-stack the raw data, which means u.data no longer contains raw values and hooks/plugins cannot read this back for display and we also have to jump through a few hoops to get the live legend to display what we want.

uPlot's current way to tweak individual datapoints for rendering is by using a user-provided disp option to the pathbuilder of choice. this then handles adapting the raw values to ones needed for actual drawing. however, currently only the bars pathbuilder supports disp*, and it only supports adjusting x0 and size, but not y, which can be used to avoid pre-processing raw data before setData() calls.

* also the scatter demo's pathbuilder

[1] https://leeoniya.github.io/uPlot/demos/stacked-series.html
[2] https://leeoniya.github.io/uPlot/demos/y-shifted-series.html

Start by reading the existing pathbuilders that support the user-provided disp option, especially the bars pathbuilder and the scatter demo's pathbuilder. Compare the stacked-series and y-shifted-series demos to understand the required y transformation. Done means disp.y can adapt values for rendering while u.data remains raw, and both demos can avoid pre-stacking or pre-shifting their data.