stepped values that are representative either before or after the most recent data point.

__Proposal:__
stepped values that are representative either before or after the most recent data point.

__Current behavior:__
Currently, selecting stepped lines in v2, the visual vertical step happens at some arbitrary location between two data points.

__Desired behavior:__
It would be nice to be able to select whether the vertical step happens at the left data point, or at the right data point.

__Alternatives considered:__
Before I add a new value to a timeseries, I first add a 'phantom point' at the current timestamp with the previous value. This simulates a vertical step at the 'new' timestamp.

__Use case:__
Why is this important (helps with prioritizing requests)?
For financial data, it makes sense that in the absense of new information, the last value seen is still valid, and the value only 'jumps' when the new information arrives.

Start by locating the v2 stepped-line rendering and the configuration that controls stepped values; no files or tests are named in the issue. Compare behavior when the vertical step represents the left data point versus the right data point, and consider the financial time-series use case. Done means users can select either interpretation and the visual step occurs at the corresponding data point.

Written by the indexing model from the issue text.