Interpolate or Fill Line Gaps

I have a chart that includes both stock candlesticks and a line. The data have gaps for weekends and holidays. (I want the gaps shown as they are.) The line connects the mean (high+low)/2 for each market day.
I'd like the mean line to connect through the blank/empty weekends and holidays. Currently the line drops to zero for empty data days. (Open the attached sample.xlsm to see the line drops.)
The mean is defined by the following, which outputs "" for non-market days:
=IFERROR((C80+D80)/2,"")
I've tried Select Data > Hidden and Empty Cells > Connect data points with line, but it doesn't do anything.

Dell XPS 8940
Desktop
Windows 11
Office Home and Student 2019
Excel v. 2201 (Build 14827.20158)

Jackpot!
Thanks Andy.

Just to tidy things up...
How do I hide the #N/A?
All of the solutions I'm seeing suggest using IFERROR to change them to zeros, which is what I don't want!

Thanks!
I solved the problem by using Wite-Out on my screen to cover the ugly cells. 🙃
(I know... most of you have never used a typewriter.)

I used the rule “=ISNA(A2:G1884)” sans quotes.

(One of my biggest problems is using the keyboard cursor arrows to move around inside of certain places. Excel interprets these keystrokes instead of moving the insertion point and messes things up.)

Your solution worked great. Thanks for all of your help.