Feature Request: reference line series

[fsher]

Similar to my previous request regarding threshold series (#174), it would be great to be able to draw similar data on the candle charts. And I understand that threshold split doesn't work well here, but having a tool to be able to do a similar thing would be great, and reference lines work here well, I believe

Thank you in advance!

[brandtnewlabs]

Implemented in #295.

ReferenceLine now accepts a sorted series of timestamped values, so a candle chart can draw historical benchmarks such as average entry cost or VWAP without using the threshold color-split treatment. The last value extends to the live edge by default; extendToNow: false stops the line at its final point.

The candlestick example now demonstrates this with a historical average-cost series. The recording below toggles it off and back on over the live candle chart.

issue-290-reference-series-proof.mp4