Charts in FileMaker are quite limited. They were a new feature released with version 11 and there's a lot of room for improvement. To do what you want, if it can be done, will require a third party charting tool at this time.
What I did:
multiply each hearing value by -1 and separate by paragraph mark (this makes numbers go down instead of up -- calculation field: Aud_L500 * -1 & ¶ & Aud_L1000 * -1 & ¶ & Aud_L2000 * -1 & ¶ & Aud_L3000 * -1, etc) );
mark each Y axis: set minimum -100, set maximum 0, show major ticks: 10; X axis are the Hrtz values (500, 1000, 2000, 3000, etc);
This will put a negative sign in front of each dB value but the graph appearance is correct.
Best I could do.
What I can't do is get the R side line Red and L side line blue.