`aboveBarData` relative cutoff

We want to make a graph with `aboveBarData` that cuts off like this.

Currently the `cutOffY` is absolute, so there isn't an easy way to do this.

Could you add a flag that `cutOffY` is relative instead of absolute. If relative and is set to "1", then it will cutoff 1 above the spot, instead of cutting off at 1 on the graph

Start by locating the chart implementation that handles `aboveBarData` and `cutOffY`, then trace how the cutoff is applied to the graph. Add a relative-cutoff option with the requested semantics, and verify that an offset of 1 cuts off one unit above the relevant spot while existing absolute behavior remains unchanged.

Written by the indexing model from the issue text.