isNaN/Double.NaN, required or failsafe?

[Nullified]

Hello, I'm very confused on the use of these NaN words. A lot of scripts use it and a lot of people on the forums are using it as well. I've seen that it may be used to prevent a function or statement from failing a condition when they can't be met (like a stock that doesn't currently have a 200D moving average). But I'm wondering, if I'm trying to create a script that only looks at the previous day or month, do I need that? I'm trying to modify the Daily High Low script now and it's in there but of course 99.99% of stocks on the market will have 1 previous day of data so I don't believe it would be necessary to check if that data is available. What could it be in there for?
________

Also, could anyone tell me how to make the lines go further? like I want the daily to go to the next day and same for the monthly.

 

[MerryDay]

The standard is to check for NaN but there is no law.

Here is eleven pages of scripts that plot lines:
https://usethinkscript.com/threads/horizontal-lines-in-thinkorswim.3472/