I have a graph where I need to find a particular point based on sampling
Based on the graph you posted and the elbow defined, all you have to do is to take the first difference of the signal and compare it to a threshold value, assuming that the signal is monotonically decreasing. Such as this:

if( (y[k] - y[k-1]) &lt; Th )
          elbow = k-1


if the signal is not monotonically decreasing, you should look for other measures as well.

Sorry but I cannot tell the required sampling rate for this graph as you stated it comes from an already discrete data...?