add relative gradient option to the baseline series

[illetid]

Type of PR: enhancement

PR checklist:

• Addresses an existing issue: fixes Baseline series should use min/max price range in visible range to calculate gradient stops #1005
• Includes tests
• Documentation update

Overview of change:
Added a new option to the baseline series where we calculate gradient from the visible points range and not from the top and bottom of the chart.
Example with two series with the same settings. Top and bottom colors are different without option enabled
without option:

with the option enabled:

[github-actions]

size-limit report 📦

Path	Size
ESM	42.42 KB (+0.16% 🔺)
ESM createChart	36.27 KB (0%)
ESM createChartEx	35 KB (-0.17% 🔽)
ESM Standalone	43.96 KB (+0.35% 🔺)
Standalone	43.88 KB (+0.3% 🔺)
Plugin: Text Watermark	1.86 KB (0%)
Plugin: Image Watermark	1.68 KB (-0.18% 🔽)
Plugin: Series Markers	3.89 KB (-0.13% 🔽)
Series: LineSeries	2.54 KB (-0.04% 🔽)
Series: BaselineSeries	3.17 KB (+3.87% 🔺)
Series: AreaSeries	3.1 KB (+3.87% 🔺)
Series: BarSeries	2.15 KB (+0.28% 🔺)
Series: CandlestickSeries	2.43 KB (-0.25% 🔽)
Series: HistogramSeries	2.2 KB (+0.18% 🔺)

[SlicedSilver]

I think we should add this option to the area series as well.

Additionally, could you check the behaviour when you have more than one series on the same price scale?
because there would be two ways to implement this feature:

• Consider the min and max visible values of items (like this PR)
• Use the min and max values from the scale margins

For a single series (and the price scale not adjusted) then the visible range and the scale margins range would essentially be the same and unchanging (and quicker to calculate).

However for multiple series, this wouldn't be the case. So please try these different setups and comment on how they look.

[SlicedSilver]

Let's maybe make the limit a bit larger.
The CI will add a comment to each PR with the size changes therefore we don't need this limit here to be so close to the actual amount but can rather be something like 4 KB

[illetid]

added the same option for the Area series. Screenshots from the PR's description represent two series on the same price scale.
I think the approach of calculating gradients based on the visible range is a better solution, because the scale margins work only for the one series and if applied margins in case if there are two or more series gradient will be shifted at the value of margin and still be inconsistent between series. with min/max values we don't care about the settings or other series on the chart, we calculate based only on the current series visible data. In case if someone has performance concerns this behavior can be disabled.
I made it false by default, WDYT, maybe it should be enabled by default?

[SlicedSilver]

I have a concern that there are graphics tests (which haven't been changed) are failing for the area series. Could you please investigate why the area series is rendering differently in these cases. The opacity seems to be different.