I found a way to enter the service menu in my Panasonic CT-26WX15. This also works for other Panasonic TV models (including CT-30WC15, CT-30WX15, and CT-34WX15). Here's how you do it:
Power on your TV
Go to menu and set antenna to CABLE
Then, go to sleep timer in the menu and set it to 30
Exit menu and go to channel 124
Then, adjust volume to 0 on the remote
Then, at the front of the TV, press the volume down and the word "CHK" in red appears
Finally, press the power button on the remote
You will then see a big service menu at the middle of the screen
To adjust, use the volume buttons to adjust the values. To change each category, use the channel buttons.
To exit the service menu, press the power button on the remote, and it will return to "CHK" in red at the upper left hand corner. Then, press the power button at the front of the TV to turn off the TV. If you want to see a summary or self-check, press the power button and action button at the same time for two seconds instead.
Don't forget to copy down all of the original values on a separate piece of paper before you adjust the values. I'm still browsing it, but I don't have the time to adjust the screen right now. It looks kind of complicated to me.
Let me know which setting can let me adjust the top horizontal line.