## 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
for j in jobs:
                j.start()
            
            for j in jobs:
                j.join()
        
        if len(errors) > 0:
            raise errors[0]
        
        res = []
        for result in res_matrix:
            if isinstance(result, List):
                res.extend(result)
            else:
                res.append(result)
        timings = []
        for result in timings_matrix:
            if isinstance(result, List):
                timings.extend(result)
            else:
                timings.append(result)
        if (
            self.query.trendsFilter is not None
            and self.query.trendsFilter.formula is not None
            and self.query.trendsFilter.formula != ""
        ):
            with self.timings.measure("apply_formula"):
                res = self.apply_formula(self.query.trendsFilter.formula, res)
        return TrendsQueryResponse(results=res, timings=timings, hogql=response_hogql)
    def build_series_response(self, response: HogQLQueryResponse, series: SeriesWithExtras, series_count: int):