Trend: Support log x and custom y scales, support for exemplars with x != time

[leeoniya]

we'd like have Trend and/or XY panel better support reliability analysis, which means Weibull probability plots that have axes scales that are log(x) and log(-log(1-y)).

https://reliability.readthedocs.io/en/latest/Probability%20plots.html

i added support to uPlot for custom scales: https://leeoniya.github.io/uPlot/demos/custom-scales.html

however, we still need to do some work in Trend panel:

• Fix support for log(x) scales on the x axis
• Add support for either custom y scales (tricky, poor UX), or specifically for Weibull plots (probably behind a feature flag due to niche use case?)
• Handle exemplar dataframes that don't have Time for x axis. this would likely require exposing a configuration which allows selecting which fields of the examplar frame should be used for x and for y.

Alternatively, we can instead support this using XYChart, which can support arbitrary x/y data points without relying on a special Exemplars frame coming through panelData.annotations. a big benefit here is that we won't have to add exemplar-related customization options to Trend, since XYChart already allows mapping of arbitrary x and y fields for each series. however, XYChart does not currently have "fillBelowTo" support for rendering band shading, and generally has a much more limited set of options for sequential/aligned data compared to Trend. ultimately it may be less effort to add series fill and fillBelowTo options to xy,