Define point size on scatter chart

[GajowyJ]

This is:

- [ ] a bug report
- [X] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

What is the expected behavior?

An user can define point size on scatter chart.

What is the current behavior?

Point size is hardcoded for 3.

What are the steps to reproduce?

Please provide a Minimal, Complete, and Verifiable example of code that exhibits the issue without relying on an external Excel file or a web server:

<?php

require __DIR__ . '/vendor/autoload.php';

// Create new Spreadsheet object
$spreadsheet = new \PhpOffice\PhpSpreadsheet\Spreadsheet();

// just create any scatter chart 

Which versions of PhpSpreadsheet and PHP are affected?

PHP 8.0.10
PhpSpreadsheet 1.18.0

[GajowyJ]

Proposed solution:

1. Add following code to Chart\DataSeriesValues.php:

class DataSeriesValues
{
	// ...
	private $pointSize;

	public function __construct( // ...
	{
		// ...
		$this->pointSize = 3;
	}

	public function getPointSize()
	{
		return $this->pointSize;
	}

	public function setPointSize($size)
	{
		$this->pointSize = $size;
		return $this;
	}
	// ...
}

2. Modify following code in Writer\Xlsx\Chart.php:

	if ($plotSeriesMarker !== 'none') {
		$objWriter->startElement('c:size');
		//$objWriter->writeAttribute('val', 3);	//  removed
		$objWriter->writeAttribute('val', $plotSeriesValues->getPointSize());	// added
		$objWriter->endElement();
	}

[bridgeplayr]

I can verify that this modification is effective. Thank you @GajowyJ