Actualy I don't think it is a problem with missing dates (i.e. gaps) in the data on which to base the time series. See the following case... OK try ------------ a datasource with a "OK_Date" field on which to base the timeserie in the graph (TimePeriodExpression). The data source is ORDERED on this field "OK_Date", and the field values aren't continuous on a daily basis. displaying the graph with a time period expressed by DAY is OK! KO try (null argument when express by day!) --------------------------------------------- using the same datasource with a "OK_Date" field . The data source is ORDERED on this field "OK_Date". BUT this time we use a variable "Display_Date" ,(validated with business logic),on which to base the timeserie in the graph (TimePeriodExpression). N.B. The "OK_Date" and "Display_Date" don't have the same chronological order. The day and month parts retain the same but some years are changed to YEAR-1. displaying the graph with a time period expressed by WEEK is OK! displaying the graph with a time period expressed by DAY is KO, with the following error: << Error filling print... Null 'period' argument. java.lang.IllegalArgumentException: Null 'period' argument.     at org.jfree.data.time.TimeSeries.addOrUpdate(TimeSeries.java:693)     at net.sf.jasperreports.charts.fill.JRFillTimeSeriesDataset.customIncrement(JRFillTimeSeriesDataset.java:146)     at net.sf.jasperreports.engine.fill.JRFillElementDataset.increment(JRFillElementDataset.java:175)     at net.sf.jasperreports.engine.fill.JRCalculator.calculateVariables(JRCalculator.java:148)     at net.sf.jasperreports.engine.fill.JRVerticalFiller.fillDetail(JRVerticalFiller.java:736)     at net.sf.jasperreports.engine.fill.JRVerticalFiller.fillReportStart(JRVerticalFiller.java:252)     at net.sf.jasperreports.engine.fill.JRVerticalFiller.fillReport(JRVerticalFiller.java:110)     at net.sf.jasperreports.engine.fill.JRBaseFiller.fill(JRBaseFiller.java:923)     at net.sf.jasperreports.engine.fill.JRBaseFiller.fill(JRBaseFiller.java:826)     at net.sf.jasperreports.engine.fill.JRFiller.fillReport(JRFiller.java:59)     at net.sf.jasperreports.engine.JasperFillManager.fillReport(JasperFillManager.java:417)     at net.sf.jasperreports.engine.JasperFillManager.fillReport(JasperFillManager.java:247)     at com.jaspersoft.ireport.designer.compiler.IReportCompiler.run(IReportCompiler.java:858)     at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:572)     at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:997) Print not filled. Try to use an EmptyDataSource... >> The only explaination I can think of is the fact that the "Display_Date" isn't chronologicaly ordered anymore (eventhought when expressed by week it is OK??). Does anyone have advices on how to resolve this  "Null 'period' argument" error? Thanks in advance