Html::cleanCssIdentifier Causes Lots of Issues

Hi,

Using Html::cleanCssIdentifier causes unintended issues. Table.php and AxisChart.php both use this method. For instance, if the $group_key starts with a number, the method will replace that number with an "_". If the $group_key is single digits, all $group_id's would end up being "_". Now, let's say you want to create multiple visualizations using the split field option, and that field contains numbers in single digits. This will only produce one chart for all single digit fields.

This appears to be an issue with v2 too.