I've been told what I'm hoping to do is called 'spline interpolation'.
I've imported monthly youtube channel viewership figures from google API. And I've layed out the dates and views in a horizontal range. On the right I want to be able to pick a date and show a rough viewership figure that's close to the actual viewership figure, so interpolating as if it were on a curved line of best fit with multiple peaks and trophs.
The reason is to create a youtube channel directory with monthly actual viewership figure and % monthly difference on a curved line of best fit since the channels beginning.
Here's the google spreadsheet example anyone can edit:
And I need it to work with blank/unknown cells. So using a formula that doesn't treat empty cells as an error, or fills them in based on the average of known figures to both sides in a series.
This is the closest answer I've found so far, but I can't get the formula/code to work: