[Feature] Set middle splitLine color on bar chart with negative value

### What problem does this feature solve?

xAxis.splitLine.lineStyle.color can set splitLine color except the middle one.

### What does the proposed API look like?

splitLine.lineStyle.color: function

Start by reproducing the bar chart case with a negative value and inspect how xAxis.splitLine.lineStyle.color is applied to the split lines, especially the middle one. Define and verify the proposed function-based color behavior, including the expected result for the middle splitLine.

Written by the indexing model from the issue text.