Line 7 is this:
It will only work if the monitor is a point!
To know the frequency points, you can use
Since usually the frequency point is in the last dimension, in your case, it is 500.
The other number 69 is the space point.
Since now it is 2D matrix, you will need to image it.
If you only want to plot phase vs f at given space point, you have to specify the space point, eg
mm=30; $the 30th space point
In addition, since now you know more about the phase extraction, please review the previous posts you got replied.