In this sentence, the key phrase is "off the charts", which is an idiom or expression. This expression is supposed to make you think of a "line chart", a measurement of something over time. For example, let us imagine we are measuring the temperature outside every day, and placing each measurement on a line graph (the chart). My chart is labeled so that the bottom of the chart is "freezing" temperature, or 0 degrees Celsius, known as 32 degrees Fahrenheit in the USA. The top of the chart is the hottest it ever gets around here, labeled 35 degrees Celsius, or 95 degrees Fahrenheit. However, this summer, one day, we measure an amazing 38 degrees Celsius (100 degrees Fahrenheit)! This is hotter than our chart actually allows us to draw, so it is "off the chart"! In your quotation, Davidson is not measuring temperature, but his happiness. He is happier than the chart expected was possible, so his "brainwaves were off the happiness charts".

In this case, "off" is not part of the verb (not a verbal phrase). So "find" in "found something to be", means discover or learn or determine (judge). Let me illustrate with a couple of examples... She found the book to be interesting. He found the dog to be friendly. Off the charts, means if you were to make a chart (graph) of some numbers, the number that you want to add to the chart is so big it won't fit on the paper. The number would be far above the paper, off the paper or off the chart. Off the happiness charts then means if you made a chart of happiness, his happiness was extremely high. We use it a lot. Let's say someone lifts weight to exercise all the time. You might say: His strength is off the charts. I found this explanation to be difficult. ;)

If something is described as "off the charts" it means that it is exceptionally high. For example, if you were talking about Usain Bolt's speed, you could say "his speed is off the charts!"

It means "Davidson was very happy." The expression "off the charts" means, approximately, "much more or much better than expected." Example: Our results were off the charts. Imagine you were marking results in a spreadsheet or on a piece of paper. If the results were so good that the marks went off the top of the page, you would be "off the chart."

In this case, "off" is not part of the verb (not a verbal phrase). So "find" in "found something to be", means discover or learn or determine (judge). Let me illustrate with a couple of examples... She found the book to be interesting. He found the dog to be friendly. Off the charts, means if you were to make a chart (graph) of some numbers, the number that you want to add to the chart is so big it won't fit on the paper. The number would be far above the paper, off the paper or off the chart. Off the happiness charts then means if you made a chart of happiness, his happiness was extremely high. We use it a lot. Let's say someone lifts weight to exercise all the time. You might say: His strength is off the charts. I found this explanation to be difficult. ;)