Difficulty extending the trend line to the bottom of a graph

tjs247  Re: ``when i change the forward period, it shifts everything upwards``

But if you look closely, the change is ony aesthetic.  That is, the data is the same relative to the values on the x and y axes.  All that changed is the range of the y-axis; specifically, 20 instead of 19.  So, of course, that will "move" the data "upward" to the new spatial position of the corresponding y-axis values.

If your only complaint is that Excel does this automagically, I'm afraid you're SOL.  Excel often applies "artificial intelligence" (read: stupidity) to make automatic aesthetic changes "on our behalf".  For example, whenever I edit formulas that use certain financial functions, Excel automagically changes the cell format to Accounting, overriding my explicit cell format.

Wish it wouldn't!  But all we can do is manually reverse the change.

In your case, you can right-click the y-axis, click Format Axis, and change Minimum and/or Maximum from Auto to Fixed, filling in your desired limits.

But don't be surprised if Excel overrides that the next time you make a change to the chart. (sigh)

It is what it is.  "Grant me the serenity to accept the things I cannot change / Courage to change the things I can / And wisdom to know the difference."