Hi,

The category series, if sharing an x axis, expect that the data be aligned. So that the category values for each series are the same, and any missing values are filled out with null or 0. They are essentially expecting tabular data so that they can corellate the data visually, as if you were looking down the rows of a table.

The chart will not attempt to corellate your data for you. If multiple series are sharing an x axis. It will use whatever you assign as the itemssource for the x axis to define the category labels, and assume that each series assigned to the axis has a value for each category on the axis, and in the same order as the categories appear on the axis.

If you don't want to plot each series on the same axis then you can create a seperate x axis for each series and each axis will have its own set of category labels and will scale its content to fill the entire chart.

But if you want to share the same category axis between series, you need to make sure your data has a value for each category and that the data for each series appears in the same order. This is easily achieved if you are using the same collection as the itemssource for every series and the axis and each series is just getting its values from a different property on the data source, but if you have a seperate collection that you are using for each series, you need to make sure that they are properly formatted.

Properly ordering and aligning the data, if there is a lot of volume, is best done in the data tier as you have various indexes to enable you to quickly relate the data and fill in missing values with null or 0 through the use of outer joins, etc. But failing that, or if there isn't significant volume, there is always an option to align the data on the client.

Here is an example of how you could build a data adapter to adjust two or more disparate collections to be aligned properly for the chart:

Xaml:

<Grid x:Name="LayoutRoot" Background="White">
        <igChart:XamDataChart x:Name="theChart">
            <igChart:XamDataChart.Axes>
                <igChart:NumericYAxis x:Name="yAxis" />
                <igChart:CategoryXAxis x:Name="xAxis"
                                       ItemsSource="{Binding Outputs[0]}"
                                       Label="{}{Label}" />
            </igChart:XamDataChart.Axes>
            
            <igChart:XamDataChart.Series>
                <igChart:ColumnSeries x:Name="c1"
                                      XAxis="{Binding ElementName=xAxis}"
                                      YAxis="{Binding ElementName=yAxis}"
                                      ItemsSource="{Binding Outputs[0]}"
                                      ValueMemberPath="Value"
                                      />
                <igChart:ColumnSeries x:Name="c2"
                                      XAxis="{Binding ElementName=xAxis}"
                                      YAxis="{Binding ElementName=yAxis}"
                                      ItemsSource="{Binding Outputs[1]}"
                                      ValueMemberPath="Value"
                                      />
            </igChart:XamDataChart.Series>
        </igChart:XamDataChart>
    </Grid>

And code behind:

public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();

            var td1 = new TestData1();
            var td2 = new TestData2();

            var da = new CategoryDataAdapter(
                (o1, o2) =>
                {
                    var s1 = (string)o1;
                    var s2 = (string)o2;
                    DateTime date1 =
                        DateTime.ParseExact(s1, "MMM",
                        CultureInfo.CurrentCulture);
                    DateTime date2 =
                        DateTime.ParseExact(s2, "MMM",
                        CultureInfo.CurrentCulture);

                    return date1.Month.CompareTo(date2.Month);
                },
                new CategoryInfo(
                    td1,
                    (o) => ((TestDataItem)o).Label,
                    (o) => new TestDataItem() { Label = (string)o, Value = 0 }),
                new CategoryInfo(
                    td2,
                    (o) => ((TestDataItem)o).Label,
                    (o) => new TestDataItem() { Label = (string)o, Value = 0 }));

            DataContext = da;
        }
    }

    public class TestData1
        : ObservableCollection<TestDataItem>
    {
        public TestData1()
        {
             Add(new TestDataItem() { Label = "Jan", Value = 2 });
             Add(new TestDataItem() { Label = "Mar", Value = 3 });
             Add(new TestDataItem() { Label = "Apr", Value = 6 });
        }
    }

    public class TestData2
      : ObservableCollection<TestDataItem>
    {
        public TestData2()
        {
            Add(new TestDataItem() { Label = "Feb", Value = 4 });  
            Add(new TestDataItem() { Label = "Mar", Value = 1 });
            Add(new TestDataItem() { Label = "Apr", Value = 8 });
        }
    }

    public class TestDataItem
    {
        public string Label { get; set; }
        public double Value { get; set; }
    }

    public class CategoryDataAdapter
    {
        private List<CategoryInfo> _inputs;
        private Comparison<object> _compareCategories;

        public CategoryDataAdapter(
            Comparison<object> compareCategories,
            params CategoryInfo[] inputs)
        {
            _inputs = new List<CategoryInfo>(inputs);
            _compareCategories = compareCategories;
            foreach (var input in _inputs)
            {
                if (input.CategoryData is INotifyCollectionChanged)
                {
                    ((INotifyCollectionChanged)input.CategoryData).CollectionChanged += 
                        CategoryDataAdapter_CollectionChanged;
                }
            }
            
            RefreshOutput();
        }

        void CategoryDataAdapter_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            //Reader Note: Something more efficient but complex could be done here.
            RefreshOutput();
        }

        private List<ObservableCollection<object>> _outputs;
        public List<ObservableCollection<object>> Outputs
        {
            get
            {
                return _outputs;
            }
        }

        private void RefreshOutput()
        {
            if (_outputs == null || _outputs.Count != _inputs.Count)
            {
                _outputs = new List<ObservableCollection<object>>();
                foreach (var input in _inputs)
                {
                    _outputs.Add(new ObservableCollection<object>());
                }
            }

            foreach (var output in _outputs)
            {
                output.Clear();
            }

            var categories = GetUniqueCategories();
            categories.Sort(_compareCategories);

            List<List<object>> inputs = new List<List<object>>();
            var indexes = new List<int>();
            foreach (var input in _inputs)
            {
                List<object> inputValues = new List<object>(input.CategoryData.OfType<object>());
                inputValues.Sort((v1, v2) =>
                    {
                        return _compareCategories(
                            input.GetCategory(v1), 
                            input.GetCategory(v2));
                    });
                inputs.Add(inputValues);
                indexes.Add(0);
            }

            for (var i = 0; i < categories.Count; i++)
            {
                for (var j = 0; j < inputs.Count; j++)
                {
                    if (indexes[j] < inputs[j].Count)
                    {
                        var currentCategory = _inputs[j].GetCategory(inputs[j][indexes[j]]);
                        if (_compareCategories(currentCategory, categories[i]) == 0)
                        {
                            _outputs[j].Add(inputs[j][indexes[j]]);
                            indexes[j]++;
                            continue;
                        }
                    }

                    _outputs[j].Add(_inputs[j].GetEmptyValueForCategory(categories[i]));
                }
            }
        }

        private List<object> GetUniqueCategories()
        {
            IEnumerable<object> allCats = null;

            foreach (var input in _inputs)
            {
                var cats = from item in input.CategoryData.OfType<object>()
                           select input.GetCategory(item);
                if (allCats == null)
                {
                    allCats = cats;
                }
                else
                {
                    allCats = allCats.Concat(cats);
                }
            }

            var uniqueCats = from cat in allCats
                             group cat by cat into g
                             select g.Key;

            return uniqueCats.ToList();
        }
    }

    public class CategoryInfo
    {
        public CategoryInfo(
            IList categoryData,
            Func<object, object> getCategory,
            Func<object, object> getEmptyValueForCategory)
        {
            _categoryData = categoryData;
            _getCategory = getCategory;
            _getEmptyValueForCategory = getEmptyValueForCategory;
        }

        private IList _categoryData;
        private Func<object, object> _getCategory;
        private Func<object, object> _getEmptyValueForCategory;
       
        public IList CategoryData { get { return _categoryData; } }
        public Func<object, object> GetCategory { get { return _getCategory; } }
        public Func<object, object> GetEmptyValueForCategory { get { return _getEmptyValueForCategory; } }
       
    }

Another option, if you have date data is to use the CategoryDateTimeXAxis which can display values at the exact date on which they occurred.
But its other semantics may not be appropriate to your scenario.

Hope this helps!

-Graham