Charts: adding and configuring 'constant lines'

I have a chart in my report what includes 'constant lines' (see image). But there seems to be a limited configuration for the 'constant lines' or at least I cannot find a ways to change the label's color, size and position. Below is the only setting that I can see for the 'constant lines' if there is anything else that I am missing please let me know.

I will appreciate any help.

Thanks,

Nataliya