How can discard "peaks" values from a chart?
I would say you have two options to choose from:


Aggregate your data
Use a low-pass-filter




Aggregating

Aggregating your data over long time periods is not cheap (whatever that means). It is a common way of displaying trends over long periods of time.

Take a look at how Bloomberg displays the exchange rate of EUR over USD.

You can filter by time period, and if you select 1 day, you see the hourly rate:



or you can filter by month, and see the rate aggregated by day. For them, the last rate is the important one, so that is the value you see. For you, however, will depend on your use case. You can choose to pick one of the following: mean, median, mode, max, min, first, last or anything else that makes sense for you.





Signal processing filtering

If you want to take a more mathematical approach to your problem, you could use a signal filtering technique to reduce high-frequencies from your signal.

What you need is a low-pass filter.

If you are using Excel, you certainly can do this.
However, I would recommend you do that in some programing language like Matlab or Python, that contain easier to use signal processing packages.