fix(scale): make time getValueForPixel the inverse of getPixelForValue

[contactjawad]

What

TimeScale.getValueForPixel (and TimeSeriesScale.getValueForPixel) is meant to be the exact inverse of getPixelForValue, but it subtracts the wrong margin offset. On a time axis with offset: true and irregularly spaced ticks — where offsets.start !== offsets.end — converting a pixel back to a value is wrong by a constant, so hover/tooltip/nearest-element interaction maps to the wrong data.

Why

getPixelForValue maps a value to a decimal using offsets.start:

return this.getPixelForDecimal((offsets.start + pos) * offsets.factor);

so the inverse must be pos = decimal / factor - offsets.start. Instead the code used offsets.end:

const pos = this.getDecimalForPixel(pixel) / offsets.factor - offsets.end;

When the two margins differ, the result is off by (start - end) * (max - min).

How

Subtract offsets.start in both src/scales/scale.time.js and src/scales/scale.timeseries.js, making getValueForPixel the exact inverse of getPixelForValue (which uses offsets.start).

Test

Added a spec in test/specs/scale.time.tests.js that builds a time axis with offset: true and irregularly spaced labels so offsets.start !== offsets.end (the test asserts this premise), then verifies the round trip getValueForPixel(getPixelForValue(v)) returns v. Before the fix the round trip is off by ~21 hours for the chosen data; after the fix it matches.