Annotation in Xamarin.iOS Chart

29 Nov 202219 minutes to read

SfChart supports annotations which allows you to mark the specific area of interest in the chart area. You can add text, images, and custom views.

The following annotations are supported in SfChart

Text annotation
Shape annotation
View annotation

Adding Annotations

You can create an instance for any type of annotations and it can be added to Annotations collection. Here for an instance, the EllipseAnnotation is added.

SFChart chart = new SFChart();
...

EllipseAnnotation annotation = new EllipseAnnotation()
{
    X1 = new NSNumber(2),

    Y1 = 35,

    X2 = new NSNumber(6),

    Y2 = 40,

    Text = "Ellipse"
};

chart.Annotations.Add(annotation);

Annotation support in Xamarin.iOS Chart

Positioning the annotation

Annotations can be positioned in plot area based on X1 and Y1 properties. For shape annotations, specify X2 and Y2 properties, if needed. The X and Y values can be specified with axis units or pixel units, and these can be identified by using the CoordinateUnit property.

Positioning based on CoordinateUnit as axis

To position the annotation based on axis, set the X1 and Y1, X2 and Y2 properties based on axis range values, if needed, set the CoordinateUnit value as Axis.

SFChart chart = new SFChart();
...

RectangleAnnotation annotation = new RectangleAnnotation()
{
    X1 = new NSNumber(4),

    Y1 = 40,

    X2 = new NSNumber(8),

    Y2 = 45,

    Text = "Axis Value",

    CoordinateUnit = ChartCoordinateUnit.Axis
};

chart.Annotations.Add(annotation);

Positioning the Xamarin.iOS Chart annotation based on axis coordinates

Positioning based on CoordinateUnit as pixels

To position the annotation based on the pixel values, set the CoordinateUnit as Pixels, and the pixel values in X1 and Y1, X2 and Y2 properties of annotation are shown in the below code snippet,

SFChart chart = new SFChart();
...

RectangleAnnotation annotation = new RectangleAnnotation()
{
    X1 = new NSNumber(1),

    Y1 = 1,

    X2 = new NSNumber(150),

    Y2 = 150,

    Text = "Pixels",

    CoordinateUnit = ChartCoordinateUnit.Pixels
};

chart.Annotations.Add(annotation);

Positioning the Xamarin.iOS Chart annotation based on pixel coordinates

Adding annotation for multiple axes

When there are multiple axes, annotation also can be added for a particular axis by using the XAxisName and YAxisName properties and it is shown in the below code snippet,

SFChart chart = new SFChart();
...

EllipseAnnotation annotation = new EllipseAnnotation()
{
    X1 = new NSNumber(4),

    Y1 = 30,

    X2 = new NSNumber(8),

    Y2 = 35,

    YAxisName = "YAxis"
};

chart.Annotations.Add(annotation);

SFSplineSeries series = new SFSplineSeries();

series.ItemsSource = new ViewModel().Data;

series.XBindingPath = "Name";

series.YBindingPath = "Value";

series.YAxis = new SFNumericalAxis()
{ 
    Name = new NSString("YAxis"),

    OpposedPosition = true,
};

chart.Series.Add(series);

Multiple axis support for annotation in Xamarin.iOS Chart

Text annotation

The TextAnnotation is used to add simple text with the help of Text property in specific points over the chart area.

SFChart chart = new SFChart();
...

TextAnnotation annotation = new TextAnnotation()
{
    X1 = 7,

    Y1 = 35,

    Text = "August"
};

chart.Annotations.Add(annotation);

Text annotation support in Xamarin.iOS Chart

Customizing text annotation

The TextAnnotation can be customized by using the LabelStyle property. The following properties are used to customize the text:

Color – Used to change the color of the text.
BackgroundColor – Used to change the background color of the text.
BorderColor – Used to change the border color.
BorderWidth – Used to change the width of the border.
Font – Used to change the text size, family, and weight.
Margin - Used to change the margin of the text.
HorizontalLabelAlignment – Used to align the text horizontally(Center,Start,End.
VerticalLabelAlignment – Used to align the text vertically(Center,Start,End.

SFChart chart = new SFChart();
...

TextAnnotation annotation = new TextAnnotation()
{
    X1 = 7,

    Y1 = 35,

    Text = "August"
};

annotation.LabelStyle.Margin = new UIEdgeInsets(5, 5, 5, 5);

annotation.LabelStyle.Font = UIFont.ItalicSystemFontOfSize(16);

annotation.LabelStyle.BorderColor = UIColor.Red;

annotation.LabelStyle.BorderWidth = 2;

annotation.LabelStyle.BackgroundColor = UIColor.FromRGB(00,80,80);

annotation.LabelStyle.Color = UIColor.White;

annotation.LabelStyle.VerticalLabelAlignment = ChartAnnotationAlignment.Start;

chart.Annotations.Add(annotation);

Customizing text annotation support in Xamarin.iOS Chart

Shape annotation

The ShapeAnnotation allows you to add annotations in the form of shapes such as rectangle, ellipse, horizontal line, vertical line, etc., at the specific area of interest in the chart area.

RectangleAnnotation – Used to draw a rectangle over the chart area.
EllipseAnnotation – Used to draw a circle or an ellipse over the chart area.
LineAnnotation – Used to draw a line over the chart area.
VerticalLineAnnotation – Used to draw a vertical line across the chart area.
HorizontalLineAnnotation – Used to draw a horizontal line across the chart area.

The following APIs are commonly used in all ShapeAnnotation:

X2 – Represents the X2 coordinate of the shape annotation.
Y2 – Represents the Y2 coordinate of the shape annotation.
FillColor – Represents the inside background color of the shape annotation.
StrokeColor – Represents the stroke color of the shape annotation.
StrokeWidth – Represents the stroke width of the shape annotation.
StrokeDashArray – Represents the stroke dashes of the shape annotation.
Text – Represents the annotation text of the shape annotation.
LabelStyle – Represents the style for customizing the annotation text of shape annotation.

Rectangle annotation

The RectangleAnnotation is used to draw a rectangle or a square in specific points over the chart area. You can customize the rounded corners of the rectangle using CornerRadius property.

SFChart chart = new SFChart();
...

RectangleAnnotation annotation = new RectangleAnnotation()
{
    X1 = new NSNumber(4),

    Y1 = 20,

    X2 = new NSNumber(6),

    Y2 = 55,
};

chart.Annotations.Add(annotation);

Rectangle annotation support in Xamarin.iOS Chart

Ellipse annotation

The EllipseAnnotation is used to draw an oval or a circle in specific points over the chart area. You can also specify the height and width of EllipseAnnotation by using the Height and Width properties.

SFChart chart = new SFChart();
...

EllipseAnnotation annotation = new EllipseAnnotation()
{
    X1 = new NSNumber(6),

    Y1 = 32,

    Height = 30,

    Width = 30
};

chart.Annotations.Add(annotation);

NOTE

When X2 and Y2 properties of EllipseAnnotation are set, Height and Width properties do not work.

Ellipse annotation support in Xamarin.iOS Chart

Line annotation

The LineAnnotation is used to draw a line in specific points over the chart area.

SFChart chart = new SFChart();
...

LineAnnotation annotation = new LineAnnotation()
{
    X1 = new NSNumber(2),

    Y1 = 35,

    X2 = new NSNumber(8),

    Y2 = 45,

    Text = "Line"
};

chart.Annotations.Add(annotation);

Line annotation support in Xamarin.iOS Chart

Adding arrow to line annotation

To display single headed arrow, set the LineCap property to Arrow. The default value of the LineCap property is None.

SFChart chart = new SFChart();
...

LineAnnotation annotation = new LineAnnotation()
{
    X1 = new NSNumber(2),

    Y1 = 40,

    X2 = new NSNumber(10),

    Y2 = 40,

    LineCap = ChartLineCap.Arrow
};

chart.Annotations.Add(annotation);

Arrow support for line annotation in Xamarin.iOS Chart

Vertical and Horizontal line annotations

The VerticalLineAnnotation and HorizontalLineAnnotation are used to draw vertical and horizontal lines in specific points over the chart area.

SFChart chart = new SFChart();
...

VerticalLineAnnotation vertical = new VerticalLineAnnotation()
{
    X1 = new NSNumber(6),
};

chart.Annotations.Add(vertical);

HorizontalLineAnnotation horizontal = new HorizontalLineAnnotation()
{
    Y1 = 35
};

chart.Annotations.Add(horizontal);

Vertical and Horizontal line annotation support in Xamarin.iOS Chart

Displaying axis label for vertical and horizontal line annotations

The VerticalLineAnnotation and HorizontalLineAnnotation display the axis labels in which the line is placed. This feature can be enabled by setting the ShowAxisLabel property to true as shown in the below code snippet,

SFChart chart = new SFChart();
...

VerticalLineAnnotation vertical = new VerticalLineAnnotation()
{
    X1 = new NSNumber(6),

    ShowAxisLabel = true
};

chart.Annotations.Add(vertical);

HorizontalLineAnnotation horizontal = new HorizontalLineAnnotation()
{
    Y1 = 35,

    ShowAxisLabel = true,
};

chart.Annotations.Add(horizontal);

Displaying axis label for vertical and horizontal line annotations in Xamarin.iOS Chart

Customizing axis label

The default appearance of the axis label also can be customized by using the AxisLabelStyle property. The following properties are used to customize the axis label:

Color - Used to change the color of the text.
BackgroundColor - Used to change the background color of the text.
BorderColor - Used to change the border color.
BorderWidth - Used to change the border width.
Font - Used to change text font size, family, and weight.
Margin - Used to set the margin for text.
LabelsPosition - Used to position the text at Inside or Outside of the chart axis. By default, the text will be positioned outside the chart axis.
LabelAlignment - Used to align the label at the start, center, or end.
CornerRadius - Used to change the corner radius of the background.
MaxWidth - Provides the maximum text width of the axis label and wraps into the next line when exceeds the maximum width.
WrappedLabelAlignment - Positions the wrapped text at the start, center, or end. The default value of the WrappedLabelAlignment property is set to Start.

SFChart chart = new SFChart();
...

VerticalLineAnnotation vertical = new VerticalLineAnnotation()
{
    X1 = new NSNumber(6),

    ShowAxisLabel = true,
};

vertical.AxisLabelStyle.Margin = new UIEdgeInsets(5, 5, 5, 5);

vertical.AxisLabelStyle.Font = UIFont.ItalicSystemFontOfSize(12);

vertical.AxisLabelStyle.BorderColor = UIColor.Blue;

vertical.AxisLabelStyle.BorderWidth = 2;

vertical.AxisLabelStyle.BackgroundColor = UIColor.Red;

vertical.AxisLabelStyle.Color = UIColor.White;

vertical.AxisLabelStyle.CornerRadius = new ChartCornerRadius(5);

chart.Annotations.Add(vertical);

HorizontalLineAnnotation horizontal = new HorizontalLineAnnotation()
{
    Y1 = 35,

    ShowAxisLabel = true
};

horizontal.AxisLabelStyle.Margin = new UIEdgeInsets(5, 5, 5, 5);

horizontal.AxisLabelStyle.Font = UIFont.ItalicSystemFontOfSize(12);

horizontal.AxisLabelStyle.BorderColor = UIColor.Blue;

horizontal.AxisLabelStyle.BorderWidth = 2;

horizontal.AxisLabelStyle.BackgroundColor = UIColor.Red;

horizontal.AxisLabelStyle.Color = UIColor.White;

horizontal.AxisLabelStyle.LabelAlignment = ChartAxisLabelAlignment.Start;

chart.Annotations.Add(horizontal);

Customizing axis label in Xamarin.iOS Chart

Adding arrow to vertical and horizontal line annotations

To display single headed arrow, set the LineCap property to Arrow. The default value of the LineCap property is None.

SFChart chart = new SFChart();
...

VerticalLineAnnotation vertical = new VerticalLineAnnotation()
{
    X1 = new NSNumber(6),

    LineCap = ChartLineCap.Arrow
};

chart.Annotations.Add(vertical);

HorizontalLineAnnotation horizontal = new HorizontalLineAnnotation()
{
    Y1 = 35,

    LineCap = ChartLineCap.Arrow
};

chart.Annotations.Add(horizontal);

Arrow support for vertical and horizontal line annotations in Xamarin.iOS Chart

Adding text in shape annotation

For all the shape annotations, the text can be displayed by using the Text property.

Customizing text in shape annotation

The text in shape annotation also can be customized by using the LabelStyle property. The following properties are used to customize the text:

Color – Used to change the color of the text.
BackgroundColor – Used to change the background color of the text.
BorderColor – Used to change the border color.
BorderWidth – Used to change the width of the border.
Font – Used to change the text size, family, and weight.
Margin - Used to change the margin of the text.
HorizontalLabelAlignment – Used to align the text horizontally.
VerticalLabelAlignment – Used to align the text vertically.

SFChart chart = new SFChart();
...

EllipseAnnotation annotation = new EllipseAnnotation()
{
    X1 = 2,

    Y1 = 30,

    X2 = 6,

    Y2 = 35,

    Text = "Ellipse"
};

annotation.LabelStyle.Margin = new UIEdgeInsets(5, 5, 5, 5);

annotation.LabelStyle.Font = UIFont.ItalicSystemFontOfSize(16);

annotation.LabelStyle.BorderColor = UIColor.Red;

annotation.LabelStyle.BorderWidth = 2;

annotation.LabelStyle.BackgroundColor = UIColor.Blue;

annotation.LabelStyle.Color = UIColor.White;

chart.Annotations.Add(annotation);

Customizing text for shape annotation in Xamarin.iOS Chart

View annotation

The ViewAnnotation allows you to add annotations in the form of own custom view with the help of View property at the specific area of interest in the chart area. The ViewAnnotation also can be aligned by using the VerticalAlignment and HorizontalAlignment properties.

SFChart chart = new SFChart();
...

ViewAnnotation annotation = new ViewAnnotation()
{
    X1 = 3,

    Y1 = 12,

    VerticalAlignment = ChartAnnotationAlignment.Start
};

UIImageView imageView = new UIImageView();

imageView.Frame = new CoreGraphics.CGRect(0,0,70,70);

imageView.Image = UIImage.FromFile("Graduate.png");

annotation.View = imageView;

chart.Annotations.Add(annotation);

View annotation type in Xamarin.iOS Chart

Annotation Visibility

The Visible property of ChartAnnotation is used to control the visibility of particular annotation. Default value of Visible property is true.

VerticalLineAnnotation verticalLineAnnotation = new VerticalLineAnnotation()
{
    Visible = false
};

chart.ChartAnnotations.Add(verticalLineAnnotation);

Event

AnnotationClicked

The AnnotationClicked event is triggered when the user has clicked the annotation. The argument contains the following information.

Annotation – used to get the instance of annotation which is clicked.
X – used to get the x position of touch point on annotation.
Y – used to get the y position of touch point on annotation..

Get the touch position in annotation

Following are the override methods that are available in annotation to send the information about touch interactions.

TouchBegan – occurs when touch down inside the annotation.
TouchMove – occurs while moving the finger inside the annotation.
TouchCancelled – called when touch cancelled on the annotation area.
TouchEnded – occurs when touch up inside the annotation.

public class TextAnnotationExt : TextAnnotation
{
   protected override void TouchBegan(NSSet touches, UIEvent uiEvent)
   {
      base.TouchBegan(touches, uiEvent);
   }

   protected override void TouchMove(NSSet touches, UIEvent uiEvent)
   {
      base.TouchMove(touches, uiEvent);
   }

   protected override void TouchCancelled(NSSet touches, UIEvent uiEvent)
   {
      base.TouchCancelled(touches, uiEvent);
   }

   protected override void TouchEnded(NSSet touches, UIEvent uiEvent)
   {
      base.TouchEnded(touches, uiEvent);
   }
}

Methods

Adding separate view for annotation

The ChartAnnotationView is used to render Annotation using the GetView method. The following code sample demonstrates how to add a separate view for annotation.

[C#]

 public class CustomEllipseAnnotation : EllipseAnnotation
        {
		    ChartAnnotationView view = new ChartAnnotationView();
            protected override ChartAnnotationView GetView()
            {
                return view;

            }
        }