## Bug description

The grouped-by-month view in charts is hella confusing. The x-axis shows a date that does not reflect the value on the y-axis.

1. the y-axis shows the number for the END of the month
2. the x-axis reads as "beginning of the month"

![CleanShot 2024-02-05 at 10 23 30@2x](https://github.com/PostHog/posthog/assets/35429197/d78df745-8319-4b78-95fa-d96f18aee357)

### Comparison with daily grouping

Compare the "group by month" view with "group by day". The x-axis labeling aligns with the y-axis values. 

![CleanShot 2024-02-05 at 10 29 20@2x](https://github.com/PostHog/posthog/assets/35429197/998825fa-7406-4214-8310-8f173980f80c)


## How to reproduce

1. Group a chart by month 
2. select "unique events"

## Proposal 

### 1. easy fix - remove the "1." from the labelling

- will still confuse readers with "is this the beginning or end of the month"

![CleanShot 2024-02-05 at 10 23 30@2x](https://github.com/PostHog/posthog/assets/35429197/1a0ae6d4-0e02-4b32-86ab-7b0e069655ed)

### 2. correct fix - show end of month

- removes ambiguity from "is that the beginning or end of month"
- harder to implement because months have different end days

![CleanShot 2024-02-05 at 10 23 30@2x](https://github.com/PostHog/posthog/assets/35429197/61226346-7f97-4bcd-87bf-53fc83c75245)


cc @jannesblobel
posthog/hogql_queries/insights/trends/trends_query_runner.py
series_order=series.series_order,
                        is_previous_period_series=True,
                        overriden_query=series.overriden_query,
                        aggregate_values=self._trends_display.should_aggregate_values(),
                    )
                )
            series_with_extras = updated_series
        return series_with_extras
    def apply_formula(self, formula: str, results: List[Dict[str, Any]]) -> List[Dict[str, Any]]:
        if self.query.trendsFilter is not None and self.query.trendsFilter.compare:
            sorted_results = sorted(results, key=itemgetter("compare_label"))
            res = []
            for _, group in groupby(sorted_results, key=itemgetter("compare_label")):
                group_list = list(group)
                series_data = map(lambda s: s["data"], group_list)
                new_series_data = FormulaAST(series_data).call(formula)
                new_result = group_list[0]
                new_result["data"] = new_series_data
                new_result["count"] = float(sum(new_series_data))
                new_result["label"] = f"Formula ({formula})"
                res.append(new_result)
            return res
        series_data = map(lambda s: s["data"], results)
        new_series_data = FormulaAST(series_data).call(formula)
        new_result = results[0]
        new_result["data"] = new_series_data
        new_result["count"] = float(sum(new_series_data))
        new_result["label"] = f"Formula ({formula})"
        return [new_result]
    def _is_breakdown_field_boolean(self):