“These readings are off the charts!”
You're watching a science fiction TV show about space explorers. The characters find a strange object in space that's releasing a lot of radiation. One character says this while looking at a computer screen.
These readings are off the charts!
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(instrument) readings
Some scientific instruments have "readings", which are the information that the equipment shows to the person using it.
(something) is off the charts
When a value is too high to be measured, you can say that it's "off the charts". For example:
The last earthquake was pretty bad, but that one we had five years ago was absolutely off the charts!
To understand this phrase, imagine that you made a graph or chart of all of the normal values of something, like the strengths of all the earthquakes that have occurred in a region. An "off the charts" value won't be able to fit onto the graph.
You can also say that a value is "off the scale".