Change the color of selected points in a scatter series

        /// <summary>
        /// Occurs when the selected items is changed.
        /// </summary>
        public event EventHandler SelectedItemsChanged;

        /// <summary>
        /// Gets or sets the selection mode.
        /// </summary>
        /// <value>The selection mode.</value>
        public SelectionMode SelectionMode { get; set; }

        /// <summary>
        /// The selected items.
        /// </summary>
        private Dictionary<int, bool> selectedItems = new Dictionary<int, bool>();

        /// <summary>
        /// Gets the indices of the selected items in this element.
        /// </summary>
        /// <returns>A collection of item indices.</returns>
        public IEnumerable<int> GetSelectedItems()
        {
            return this.selectedItems.Keys;
        }

        /// <summary>
        /// Clears the selected items.
        /// </summary>
        public void ClearSelectedItems()
        {
            this.selectedItems.Clear();
            this.OnSelectedItemsChanged();
        }

        /// <summary>
        /// Determines whether the specified item is selected.
        /// </summary>
        /// <param name="index">The index of the item.</param>
        /// <returns><c>true</c> if the item is selected; otherwise, <c>false</c>.</returns>
        public bool IsItemSelected(int index)
        {
            return this.selectedItems.ContainsKey(index);
        }

        /// <summary>
        /// Selects the specified item.
        /// </summary>
        /// <param name="index">The index.</param>
        public void SelectItem(int index)
        {
            this.selectedItems[index] = true;
            this.OnSelectedItemsChanged();
        }

        /// <summary>
        /// Unselects the specified item.
        /// </summary>
        /// <param name="index">The index.</param>
        public void UnselectItem(int index)
        {
            if (this.selectedItems.ContainsKey(index))
            {
                this.selectedItems.Remove(index);
                this.OnSelectedItemsChanged();
            }
        }

        /// <summary>
        /// Raises the <see cref="E:SelectedItemsChanged" /> event.
        /// </summary>
        /// <param name="args">The <see cref="EventArgs"/> instance containing the event data.</param>
        private void OnSelectedItemsChanged(EventArgs args = null)
        {
            var e = this.SelectedItemsChanged;
            if (e != null)
            {
                e(this, args);
            }
        }

    public enum SelectionMode { All, Single, Multiple }