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.
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.