Hi Kwanghun,
You can use "annotation" to place text on your figure.
Here is an example:
% Just setting up a sample image, your image would replace this section
>> figure
>> imshow('peppers.png');
% You can set this to be any value you would like,
% depending on how your violet line value is represented in your code
>> valueOfVioletLine = 27.3;
% Making the Annotation
>> annotation('textbox',[.2 .2 .3 .3],'String',valueOfVioletLine,'LineStyle','none')
You can change the annotation's properties quite a bit to customize the appearance. See the following documentation links for more information.
