What you could do is render your own label on top of the Chart as the background will be transparent and not hide parts of the chart. However, you will need to figure out a way to know at which height to render the label. You could get a measurement of the Chart's height and determine an offset from the bottom (or top) that would bet close to your annotation.
For example, if you knew your data has a maximum value of 10, a minimum value of 0, and that you place your annotation at 7. You can calculate an offset that is 70% of the height of the chart (plus or minus where you want to position the label in respect to the annotation).
I've built you a simple demo to convey the idea (find ti attached). You would need to tweak this to meet your needs, but I think it gets the point across on how you can position a label with a little math:
// The Annotation definition
var lineAnnotation = new CartesianGridLineAnnotation()
Axis = chart.VerticalAxis,
Value = 7,
Stroke = Color.Blue,
StrokeThickness = 2
// 1 - Get the value to offset the label by doing a little math on the annotation's value against the vertical axis's range
// this would be 10 because my min is 0 and max is 10, but make sure your math is robust enough to catch divide by zero and other related exceptions
var range = verticalAxis.Maximum - verticalAxis.Minimum;
var annotationPosition = (int)lineAnnotation.Value;
var percentOffset = annotationPosition / range;
// 2 - now, with the percentage to offset, calculate where to place the Label
// (you can optimize the math for this, I am breaking down each step for visibility)
var offsetFromBottom = chart.HeightRequest * percentOffset;
var offsetFromTop = chart.HeightRequest - offsetFromBottom;
// 3 - Calculate the actual offset for the Label using it's FontSize and minor adjustment so that it's not rendered directly on top of the Annotation
var labelYOffset = offsetFromTop - AnnotationLabel.FontSize - 10;
// 4 - Finally, apply the values to the Label
AnnotationLabel.Margin = new Thickness(40, labelYOffset, 0, 0);
AnnotationLabel.Text = "PFA";
AnnotationLabel.IsVisible = true;
Screenshots at runtime (UWP):
Lance | Tech Support Engineer, Sr.
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