why simulink plot nothing when scalar input is given from image processing part and plot with the same scalar input when given from a constant block

Hi! I am working on 'autonomous navigation system' in simulink.I have divided my work into two parts.Image processing part and control system part. Lane position information which is a scalar value e.g 6 is taken through image processing part and given as input to pid.I am intially doing it with an image and the issue is when this value from image is given to pid there is no plot but when I give a mathematical value from constant block pid works perfectly.How to resolve this issue please help me.