Optimize static elements on both data and element id

[jlstevens]

Currently only the data id is used in this optimization. If both the element and the data id were used, I think it would still work where it needs to without causing confusing behavior when working with mutable data.

[jlstevens]

After discussing this with Philipp we decided we might want something like this:

def plot_id(obj):
   if hasattr(obj, '_plot_id'): 
      return obj._plot_id
   else: 
      return id(obj)

And then the clone method can transfer the parent _plot_id to the clone output. With something like:

   if self._plot_id is not None:
      clone._plot_id = self._plot_id

Hopefully this will allow the static source optimization to work better without worrying about the various uses of clone in the plotting code.

[philippjfr]

The fix described above has now been merged.