CustomLegendItem Class

An individual item in a legend that explains point or line/area segment colors for a series painted with a point or segment colorizer.

Namespace: DevExpress.Maui.Charts

Assembly: DevExpress.Maui.Charts.dll

NuGet Package: DevExpress.Maui.Charts

Declaration

public class CustomLegendItem :
    ChartElement

The following members return CustomLegendItem objects:

Example

In this example, the area chart visualizes the visible light spectrum.

Area Segment Colorizer

Create a data source - a list of objects that define data points with numeric arguments (wavelength) and values (intensity).

Create a colorizer class (LightSpectorColorizer, in this example) that implements the ICustomPointColorizer interface.
Implement the GetColor method to return a data point’s color based on a ColoredPointInfo object passed to the method as a parameter. Use the ColoredPointInfo.NumericArgument property to obtain the data point’s argument value.
This class should also implement the ICustomPointColorizer.GetLegendItemProvider method and the ILegendItemProvider interface to specify legend items that show wavelength colors.

using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using DevExpress.Maui.Charts;
using Microsoft.Maui.Controls;
using Microsoft.Maui.Graphics;

namespace SegmentColorizerExample {
    public class ViewModel {
        LightSpectorData data = new LightSpectorData();
        public IList<NumericData> LightSpectorData => this.data.LightSpectors;
    }

    public class LightSpectorData {
        public IList<NumericData> LightSpectors { get; }

        public LightSpectorData() {
            LightSpectors = new List<NumericData>();
            using (Stream stream = 
                GetType().Assembly.GetManifestResourceStream("SegmentColorizerExample.LightSpector.dat")) {
                StreamReader reader = new StreamReader(stream);
                string data = reader.ReadToEnd();
                String[] dataItems = data.Split(new String[] { "\n" }, 
                                     StringSplitOptions.RemoveEmptyEntries);
                for (int i = 0; i < dataItems.Length; i++) {
                    String[] row = dataItems[i].Split(new String[] { " " }, 
                                   StringSplitOptions.RemoveEmptyEntries);
                    double argument = Convert.ToDouble(row[0], CultureInfo.InvariantCulture);
                    double value = Convert.ToDouble(row[1], CultureInfo.InvariantCulture);
                    LightSpectors.Add(new NumericData(argument, value));
                }
            }
        }
    }

    public class LightSpectorColorizer : ICustomPointColorizer, ILegendItemProvider {
        Color waveLengthToColor(double waveLength) {
            double gamma = 0.8;
            double red = 0.0;
            double green = 0.0;
            double blue = 0.0;
            if (waveLength <= 440) {
                double attenuation = 0.3 + 0.7 * (waveLength - 380) / (440 - 380);
                red = Math.Pow((-(waveLength - 440) / (440 - 380)) * attenuation, gamma);
                green = 0.0;
                blue = Math.Pow(1.0 * attenuation, gamma);
            }
            else if (waveLength <= 490) {
                red = 0.0;
                green = Math.Pow((waveLength - 440) / (490 - 440), gamma);
                blue = 1.0;
            }
            else if (waveLength <= 510) {
                red = 0.0;
                green = 1.0;
                blue = Math.Pow(-(waveLength - 510) / (510 - 490), gamma);
            }
            else if (waveLength <= 580) {
                red = Math.Pow((waveLength - 510) / (580 - 510), gamma);
                green = 1.0;
                blue = 0.0;
            }
            else if (waveLength <= 645) {
                red = 1.0;
                green = Math.Pow(-(waveLength - 645) / (645 - 580), gamma);
                blue = 0.0;
            }
            else if (waveLength <= 750) {
                double attenuation = 0.3 + 0.7 * (750 - waveLength) / (750 - 645);
                red = Math.Pow(1.0 * attenuation, gamma);
                green = 0.0;
                blue = 0.0;
            }
            return Color.FromRgb(red, green, blue);
        }

        protected virtual double[] GetWavesForLegend() {
            return new double[] { 400, 440, 480, 540, 580, 610, 650 };
        }

        Color ICustomPointColorizer.GetColor(ColoredPointInfo info) {
            return waveLengthToColor(info.NumericArgument);
        }
        ILegendItemProvider ICustomPointColorizer.GetLegendItemProvider() {
            return this;
        }

        CustomLegendItem ILegendItemProvider.GetLegendItem(int index) {
            double waveLength = GetWavesForLegend()[index];
            Color color = waveLengthToColor(waveLength);
            return new CustomLegendItem($"{waveLength} nm", color);
        }
        int ILegendItemProvider.GetLegendItemCount() {
            return GetWavesForLegend().Length;
        }
    }

    public class NumericData {
        public double Argument { get; private set; }
        public double Value { get; private set; }
        public NumericData(double argument, double value) {
            Argument = argument;
            Value = value;
        }
    }
}

Assign a SegmentBasedFillColorizer object to the AreaSeries.FillColorizer property.

Set the SegmentBasedFillColorizer.SegmentColorizer property to a GradientPointBasedSegmentColorizer object, and assign a LightSpectorColorizer object to the GradientPointBasedSegmentColorizer.PointColorizer property.

XAML

<ContentPage
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:dxc="clr-namespace:DevExpress.Maui.Charts;assembly=DevExpress.Maui.Charts"
    x:Class="SegmentColorizerExample.MainPage"
    xmlns:local="clr-namespace:SegmentColorizerExample">
    <ContentPage.BindingContext>
        <local:ViewModel/>
    </ContentPage.BindingContext>
    <dxc:ChartView>
        <dxc:ChartView.Series>
            <dxc:AreaSeries>
                <!-- Bind the series to the data source. -->
                <dxc:AreaSeries.Data>
                    <dxc:SeriesDataAdapter DataSource="{Binding LightSpectorData}" 
                                           ArgumentDataMember="Argument">
                        <dxc:ValueDataMember Type="Value" Member="Value"/>
                    </dxc:SeriesDataAdapter>
                </dxc:AreaSeries.Data>

                <dxc:AreaSeries.FillColorizer>
                    <dxc:SegmentBasedFillColorizer>
                        <dxc:SegmentBasedFillColorizer.SegmentColorizer>
                            <dxc:GradientPointBasedSegmentColorizer 
                                PointColorizer="{local:LightSpectorColorizer}"/>
                        </dxc:SegmentBasedFillColorizer.SegmentColorizer>
                    </dxc:SegmentBasedFillColorizer>
                </dxc:AreaSeries.FillColorizer>

            </dxc:AreaSeries>
        </dxc:ChartView.Series>

        <!-- Specify axis and legend settings here. -->
    </dxc:ChartView>
</ContentPage>