Vertical grid lines in bar graphs

`BarCharts` seem to ignore vertical lines set up in their `FlGridData`.

I understand from https://github.com/imaNNeoFighT/fl_chart/issues/92 that this was removed on purpose:
```
we decided to remove the horizontal value concept from the BarChart. That's why we don't show anything.
Feel free to make a new issue to follow up on this decision. I need to understand the use case (Because I think in the barChart the only yAxis is important).
```

So here's my use case: I've got a screen with several graphs, line and bar, and they all describe the same time range, e.g. last quarter with one datapoint per week. I'd like to put vertical lines in the grid separating *months* which do not always fall between weeks. It works great in LineChart, but it does not with a BarChart using identical FlGridData.

Start with the BarChart handling of FlGridData and compare it with the LineChart behavior, using the issue's identical configuration as the reproduction. Trace how vertical grid lines are positioned relative to bar-chart x values. Done means vertical lines can render at requested positions, including month boundaries that fall between weekly data points.

Written by the indexing model from the issue text.