GraphNG: refactor by-value color schemes (#37670)

* GraphNG: account for top canvas padding in gradient gen for color scheme/thresholds-by-value

* Updated test dashboard

* Added fix for issue when scaleMin was same as threshold

* fixed firefox issue

* revert docs changes

* update gdev dash for easier comparisons & regression spotting

* refactor

* optimize gradient re-gen/re-use and color more tinycolor.setAlpha() -> alpha(). update uPlot to dev build.

* fix percentage steps

* implement % threshold region rendering

* crisp threshold line rendering

* simplify

* WIP: hoverpoint dynamic color interpolation

* fix hover point color interp

* re-use gradient gen to draw threshold areas

* re-implement by-value color scales

* tweak comment

* mimic tinycolor behavior in colorManipulator.alpha() for empty colors

* explicitly disable hover points for BarChart and Histogram

* reduce test failures and required changes to tests

* fix barchart tests

* uPlot 1.6.15

Co-authored-by: Torkel Ödegaard <torkel@grafana.com>
Co-authored-by: Ryan McKinley <ryantxu@gmail.com>

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