Get an artist chart for a user profile, for a given date range. If no date range is supplied, it will return the most recent artist chart for this user.
user (Required) : The last.fm username to fetch the charts of. from (Optional) : The date at which the chart should start from. See User.getWeeklyChartList for more. to (Optional) : The date at which the chart should end on. See User.getWeeklyChartList for more. api_key (Required) : A Last.fm API key.