anchor.diff:          "period" {
               "from" : "From when the artifacts are displayed",
               "to" : "Moment until artifacts are displayed",
  -            "category" : "The category providing the timestamp to use"
  +            "category" : "The category id providing the timestamp to use"
           },
           "filters" : [{
               "category" : "Filter the artifact with the specified category",
  
positive.code:     }
  
  * `unit`: the time unit that corresponds to the group level of the time series from year to minute. This field is mandatory. Possible values are `year`, `year/month`, `year/month/day`, `year/month/day/hour`, `year/month/day/hour/minute`.
  * `category`: the category `tag` defining how `Artifact` must be grouped. This field is mandatory.
  * `preferences.color`: an array where each object describes the preferred color to represent a category. Each object has a `label` attribute for the category label and a `value` for the color to apply.
  * `prefereces.chart`: a string indicated the chart displaying the data. The possible values are managed by the user interface.
  
  ==== event = 'REMOVE_VIEW'
  
  The `REMOVE_VIEW` event for an `Organization` has a `data` attribute associated to an empty value.
  In fact, the event simply indicates that a view identified by the event `tag` has been removed.
  Therefore, no more information than the `tag` is required to identify the removed view.
  
  === Artifact
  
  
negative.code:         "rows" : "Category enumerating the rows"
      }
  
  * `columns`: a category `id` containing the values displayed in column titles. `Artifact` will be organized vertically according to the value of this category. This value is mandatory.
  * `rows`: a category `id` containing the values displayed in row titles. `Artifact` will be organized horizontally according to the value of this category. The value can be `undefined`, `null` or `false`. In this case the table has only one dimension.
  	
  Additional attributes when `type = 'timeseries'`.
  
  