You can add a reference line to your Heatmap Object.
Determine the Dimension Value.
Define the Label for your Reference Line.
Choose the Font Color of the Label. (Use the color-palette or set the expression.)
Define the Font Size of the Label. (Slider or by expression.)
Change the transparency of the Label.
Choose between bold, underlined, italic Font Style.
Choose between top and bottom position of the Label.
Choose between left/ center/ right horizontal alignment of the Label.
Set the Reference Line Color.
Define the Thickness of your Reference Line.
Determine the Visibility of the Reference Line.
You can make the line dashed or normal.
Determine Line Position : Top/ Middle/ Bottom.
Condition for the reference line to be displayed only when the calculation condition is true.
heatmap with reference lines example: