Add optional id to metrics

[burkmarr]

To overcome a problem with highlighting (and transitions?) on charts where metrics can be dynamically added and removed, I added an optional id property to temporal chart. This is to uniquely id metric - the name of the property on which metric is based may not always be sufficient because more than one metric can be based on same property which is why code adds property index to property. But the indexes change as metrics are added/removed. Hence the need for unique ID under these circumstances.

This issue is to remind that the same fix might need to be added to other chart types.