It seems you're confused both about interpolation/extrapolation as well as how to read a log graph. You can easily figure out the value for anything below 20kHz on this graph, which is the farthest right grey line. For example, to figure out where 15kHz lies, you can calculate (log(15000)-log(10000))/(log(20000-log(10000)) which gives you 0.585, so you know 15kHz is 58.5% of the way between the 10kHz and 20kHz line (which gives you something like -1.6dBs). Another example, 13kHz is 37.9% of the way from 10kHz to 20kHz on the log graph, so it is around -1.3dB. This isn't interpolation or extrapolation, it's just reading the graph at known points. (technically, you're interpolating the point at which you're reading the graph from, but you're not interpolating/extrapolating the value on the graph).
As for not knowing enough about DACs to read a graph... well...