Added optional parameter 'id' to JSON

Added optional parameter 'id' to Jit BarChart, because the previous implementation was not allowing two or more same labels.
If an id is not specified, it is using key => value, where key is the label, and for two same labels it would not count the second one.
If an id is specified, it will use id => value.

E.g. one can use the following JSON code to display the same label:
'values': [
{
  'id':'a',
  'label': 'date A',
  'values': [20, 40, 15, 5]
}, 
{
  'id':'b',
  'label': 'date A',
  'values': [30, 10, 45, 10]
}]