extract some generic patterns for non-line charts

while working on #179, i'm having to copy and slightly tweak some code and noticing patterns that could be generally extracted regarding how data can/should be represented and translated to plotting custom points (which is how bars, candlesticks and boxes are implemented).

boxes and candlesticks

• data layout - each component of the box is plotted as a separate series. this is needed for uPlot to properly do vertical ranging.
• the fact that each drawn datapoint is a composite of multiple series and all drawing at the series level must be disabled.
• the box drawing code itself is basically identical, with only the box fill changing based on the given data index (in candlesticks) and needing to draw a median bar + extremes cross-bars for the box-plot case.
• the scales.x.distr type: 2, to snap to exact bounds during zooming.
• the plugins use the draw hook to draw the points.
• re-use the legend as a tooltip
• draw a "hovered" background under the currently-displayed datapoint.

a generic box/composite-point base plugin can:

• accept a callback that's fired per point to set the box styling or other metrics.
• accept a callback that's fired per point to draw extra decorations, like median bar, extreme cross-bars or mean
• rather than using the draw hook, can use the currently-unused series[0].points to draw the composite points from multiple series. this might require a slight tweak to the series draw loops which skip series[0].

bars

• handles properly distributing/spreading the series data from a single index horizontally (and centering the whole group over the x point). it also handles adding padding to the chart and disabling zoom and cursor interaction.
• draws custom bars points

a generic plugin here would be one that handles hz spreading and x scale padding. then accepting a custom point drawing function. (e.g. bars).

Start by comparing the existing boxes, candlesticks, and bars implementations described in the issue, especially their plugins, draw hooks, series[0].points handling, and series draw loops. Determine whether a generic composite-point plugin and a separate bar-spreading plugin can cover those patterns; done means the repeated behavior is consolidated without changing the existing chart types.