Version 2.0.0

Changes:

• Several fixes to work with php8
• Add showEmptyYValues property
• Add cssID field to chart content element
• Fix format string for y axis
• Set yLabelCount
• Add option to display second y axis
• Add first version of custom tooltip implementation
• Add basic styling for custom tooltip
• Add subchart functionality
• Add option for grid lines
• Add missing sorting entry
• Decode html entities in chart titles
• ...