/// <summary>
    /// SelectedColumnChanged event delegate.
    /// </summary>
    /// <param name="sender">Object triggering the event.</param>
    /// <param name="e">Event arguments.</param>
    public delegate void SelectedColumnChangedEventHandler(object sender, SelectedColumnChangedEventArgs e);

    /// <summary>
    /// SelectedColumnChanged event arguments.  Contains the old and the newly 
    /// selected column.
    /// </summary>
    public class SelectedColumnChangedEventArgs : EventArgs
    {
        /// <summary>
        /// Old selected column.
        /// </summary>
        public ColumnItem OldColumn { get; private set; }

        /// <summary>
        /// New selected column.
        /// </summary>
        public ColumnItem NewColumn { get; private set; }

        /// <summary>
        /// Constructor.
        /// </summary>
        /// <param name="oldColumn">Old selected column.</param>
        /// <param name="newColumn">New selected column.</param>
        public SelectedColumnChangedEventArgs(ColumnItem oldColumn, ColumnItem newColumn)
        {
            OldColumn = oldColumn;
            NewColumn = newColumn;
        }

    }

    /// <summary>
    /// Extended OxyPlot ColumnSeries class.  Allows selection of columns and tracks which 
    /// column has been selected.
    /// </summary>
    public class SelectableColumnSeries : ColumnSeries, INotifyPropertyChanged
    {
        /// <summary>
        /// Selected ColumnItem field.
        /// </summary>
        private ColumnItem _selectedColumn;

        /// <summary>
        /// Selected ColumnItem property.
        /// </summary>
        public ColumnItem SelectedColumn
        {
            get { return _selectedColumn; }
            set
            {
                if (_selectedColumn != value)
                {
                    var changeArgs = new SelectedColumnChangedEventArgs(_selectedColumn, value);
                    _selectedColumn = value;
                    NotifyPropertyChanged();
                    NotifySelectedColumnChanged(changeArgs);
                }
            }
        }

        /// <summary>
        /// Selected ColumnItem color field.
        /// </summary>
        private OxyColor _selectedColumnColor;

        /// <summary>
        /// Selected ColumnItem color property.
        /// </summary>
        public OxyColor SelectedColumnColor
        {
            get { return _selectedColumnColor; }
            set
            {
                _selectedColumnColor = value;
                NotifyPropertyChanged();
            }
        }

        /// <summary>
        /// SelectableColumnSeries constructor.
        /// </summary>
        public SelectableColumnSeries()
        {
            MouseDown += SelectableColumnSeries_MouseDown;
        }

        /// <summary>
        /// MouseDown event handler for the SelectableColumnSeries.  Handles changing of the selected 
        /// ColumnItem, as well as its color, if a SelectedColumnColor was provided.  If the color for 
        /// selection was not provided, then no visual feedback is provided, but the selection is still 
        /// made.
        /// </summary>
        /// <param name="sender">Event sender (object).</param>
        /// <param name="e">Event arguments (OxyMouseDownEventArgs).</param>
        protected void SelectableColumnSeries_MouseDown(object sender, OxyMouseDownEventArgs e)
        {
            var nearest = GetNearestPoint(e.Position, false);
            var column = nearest.Item as ColumnItem;
            if (column == null)
            {
                return;
            }

            if (SelectedColumn != null)
            {
                SelectedColumn.Color = FillColor;
            }

            if (!SelectedColumnColor.IsUndefined())
            {
                column.Color = SelectedColumnColor;
            }

            SelectedColumn = column;
        }

        /// <summary>
        /// PropertyChanged event handler.
        /// </summary>
        public event PropertyChangedEventHandler PropertyChanged;

        /// <summary>
        /// PropertyChanged event handling method.
        /// </summary>
        /// <param name="propertyName">Property name.  If not passed, name of the property, 
        /// which called this method, is used.</param>
        private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
        {
            if (PlotModel != null)
            {
                PlotModel.InvalidatePlot(true);
            }

            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        /// <summary>
        /// SelectedColumnChanged event handler.
        /// </summary>
        public event SelectedColumnChangedEventHandler SelectedColumnChanged;

        /// <summary>
        /// SelectedColumnChanged handling method.
        /// </summary>
        /// <param name="e">Event arguments.  Includes old column and the newly selected column.</param>
        protected virtual void NotifySelectedColumnChanged(SelectedColumnChangedEventArgs e)
        {
            if (SelectedColumnChanged != null)
            {
                SelectedColumnChanged(this, e);
            }
        }
    }

    /// <summary>
    /// Column graph view model.
    /// </summary>
    public class ColumnGraphViewModel : ViewModelBase
    {
        /// <summary>
        /// Graph item observable collection field.
        /// </summary>
        private ObservableCollection<ExtendedColumnItem> _items;
        /// <summary>
        /// Graph item observable collection property.
        /// </summary>
        public ObservableCollection<ExtendedColumnItem> Items
        {
            get { return _items; }
            set { Set(() => Items, ref _items, value); }
        }

        /// <summary>
        /// Plot model field.
        /// </summary>
        private PlotModel _model;
        /// <summary>
        /// Plot model property.
        /// </summary>
        public PlotModel Model
        {
            get { return _model; }
            set { Set(() => Model, ref _model, value); }
        }

        /// <summary>
        /// Linear axis field.  This is the axis on the left side, which 
        /// provides a reference for numerical values.
        /// </summary>
        private LinearAxis _valueAxis;
        /// <summary>
        /// Linear axis property.  This is the axis on the left side, which 
        /// provides a reference for numerical values.
        /// </summary>
        public LinearAxis ValueAxis
        {
            get { return _valueAxis; }
            set { Set(() => ValueAxis, ref _valueAxis, value); }
        }

        /// <summary>
        /// Category axis field.  This is the axis on the bottom, which 
        /// provides categories for the columns.
        /// </summary>
        private CategoryAxis _bottomCategoryAxis;
        /// <summary>
        /// Category axis property.  This is the axis on the bottom, which 
        /// provides categories for the columns.
        /// </summary>
        public CategoryAxis BottomCategoryAxis
        {
            get { return _bottomCategoryAxis; }
            set { Set(() => BottomCategoryAxis, ref _bottomCategoryAxis, value); }
        }

        /// <summary>
        /// Column series field.
        /// </summary>
        private SelectableColumnSeries _series;
        /// <summary>
        /// Column series property.
        /// </summary>
        public SelectableColumnSeries Series
        {
            get { return _series; }
            set { Set(() => Series, ref _series, value); }
        }

        /// <summary>
        /// Default constructor.
        /// </summary>
        public ColumnGraphViewModel(List<ExtendedColumnItem> items,
            string graphTitle = "", string valueAxisTitle = "", string categoryAxisTitle = "")
        {
            Initialize(items, graphTitle, valueAxisTitle, categoryAxisTitle);
        }

        /// <summary>
        /// Initializes all of the properties.  May be overridden.
        /// </summary>
        private void Initialize(List<ExtendedColumnItem> items,
            string graphTitle = "", string valueAxisTitle = "", string categoryAxisTitle = "")
        {
            Items = new ObservableCollection<ExtendedColumnItem>(items);

            Model = new PlotModel
            {
                Title = graphTitle,
                TitleColor = OxyColor.FromArgb(200, 0, 0, 0),
                IsLegendVisible = false
            };

            BottomCategoryAxis = new CategoryAxis
            {
                Title = categoryAxisTitle,
                AxisTitleDistance = 10,
                TitleFontSize = 13,
                ItemsSource = Items,
                LabelField = "Label",
                IsZoomEnabled = false,
                IsPanEnabled = false,
                GapWidth = .2

            };
            Model.Axes.Add(BottomCategoryAxis);

            ValueAxis = new LinearAxis
            {
                Title = valueAxisTitle,
                AxisTitleDistance = 10,
                TitleFontSize = 13,
                Position = AxisPosition.Left,
                MinimumPadding = 0,
                AbsoluteMinimum = 0,
                IsZoomEnabled = false,
                IsPanEnabled = false
            };
            Model.Axes.Add(ValueAxis);

            Series = new SelectableColumnSeries
            {
                ValueField = "Value",
                StrokeThickness = 1,
                LabelPlacement = LabelPlacement.Inside,
                LabelFormatString = "{0}",
                TextColor = OxyColor.FromArgb(255, 255, 255, 255),
                FillColor = OxyColor.FromArgb(200, 0, 121, 211),
                SelectedColumnColor = OxyColor.FromArgb(200, 76, 179, 255),
                TrackerFormatString = "{1}: {Value}"
            };
            Series.Items.AddRange(Items);
            Model.Series.Add(Series);
        }
    }