Displaying the Value of a Specific Point on a Trendline

Isn't this what FORECAST() does?

Given an array of values (e.g. your monthly expenditures) and an array of indices (e.g. months of the year) FORECAST() will return an expected value for a given period:

For example, I replicated your table (at least what I could wean from your description).

I added an additional column B which calculates MONTH(A) to extract the month number (FORECAST() can't mix date and numeric fields, so I just extract the relevant month data.

Now the formula in the footer cell is:

=FORECAST(12,C2:C13,B2:B13)

This calculates the forecasted 12th value based on the expenses in C2:13, based on the indexes in B2:B13 (months). In this case, based on the expenses listed, it forecasts my December expense will be $154.06

> I was using the date as one of the arrays not the month so I just put in a month column like yours that I can hide.

There is a way to do it - it's just a little more work, but may be better depending on the consistency of the dates (e.g. the expenses are paid on the first of every month, then the month index is sufficient, but if the expenses vary, then you may want a specific date).

The solution in this way would be to use the date column (rather than month index) as the third parameter to FORECAST(), and provide a date value as the first parameter, like:

=FORECAST(DATE(2025,12,1),C,A)

which may be more flexible, depending on circumstances.