Feature: Allow custom X-Axis labeling

[markusressel]

This PR adds the ability to draw custom X-Axis labels:

To use this, a dev must provide a custom function that calculates the X-Axis label for an index:

bmLineChart.SetXAxisLabelFunc(func(i int) string {
    label := fmt.Sprintf("%.1f", xFunc1(i))
    return label
})

Please have a look at the added example in demos/plot_xaxis_labels/main.go for a better explanation. By default this function simply returns i as a string, maintaining the existing behavior.

Animated Version:

Note that it is automatically determined which labels are shown, based on the available space and the width of each individual label. Labels are selected based on their centered distance to the actual value in the graph.

[markusressel]

Note: The Example is not complete yet, since it is supposed to use the functionality of #68 . As with #68 , please let me integrate these changes before merging.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 22.58065% with 48 lines in your changes missing coverage. Please review.

Project coverage is 51.08%. Comparing base (40e5944) to head (091fcae).

Files with missing lines	Patch %	Lines
plot.go	22.58%	44 Missing and 4 partials :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #69 +/- ## ========================================== - Coverage 53.61% 51.08% -2.54% ========================================== Files 9 9 Lines 968 1016 +48 ========================================== Hits 519 519 - Misses 433 477 +44 - Partials 16 20 +4 ```

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[navidys]

Hi @markusressel

Thanks for the PR, I have merged the previous PRs, will wait for you to push your changes to this PR and then will release a new version.

Thanks.

[markusressel]

@navidys Necessary changes have been made, this is how it looks now:

[markusressel]

@navidys I have fixed as many issues as I could, please decide yourself what to do with the rest.

[markusressel]

Also: I do not plan to create any more feature PRs after this for the time beeing ;)

[navidys]

Hi @markusressel Thanks for the PR, I will fix the lint and merge it.