[BUG] `Chart` class is not correctly cleaned up at plotting time

[fenix-hub]

Describe the bug Calling the plot() function should completely clear the chart (points, datastructure and grid), to let multiple plot calls behave correctly. This is what is happening currently

To Reproduce Steps to reproduce the behavior:

Expected behavior A clear and concise description of what you expected to happen.

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Desktop (please complete the following information):

• OS: [e.g. Windows]
• Godot Engine Version [e.g. 3.2.3rc6]
• Plugin Version [e.g. v1.7.0]

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[JFerrerBeired]

This should have been fixed with the last commit on the scatter branch.