What's the word for a segment/part of a spectrum chart?

I don't know of any term that applies to any spectral chart, but this is apparently the result of a radiated emissions test for electromagnetic compliance. In this case we can say,

• The test failed due to emissions between 300 and 400 MHz.

• Emissions exceeded the test limits between 300 and 400 MHz.

• 300 to 400 MHz radiated emissions are exceedingly high.

For the case where " it's not a range but a specific point, for example, to point out 300 [word] being the highest", you could say,

• Peak emissions were observed at 300 MHz.

More informally, you might just say,

• The highest peak was at 300 MHz.

Edit

The term band was offered up in the comments, and this could probably be used in many instances. But in my experience a band is more often a range of frequencies that's defined according to an application (for example "Ka-band radar" or the "10-meter amateur radio band") rather than one defined in reference to an experimental result.