How to determine a waveform graph's threshold

[jake83]

I have plotted a graph of Power(mW)(y) vs. Current (mA)(x). The graph progressively rises to reach a steady slope. My question is how do i determine the intersection of this steady sloped part of the graph with the x-axis which would altimately determine the threshold Current.

[djadjok]

Why not to use linear fitting vi's on your sloped part of data? As a result you'll have your interception coefficients. To find what part of your data are "sloped" the analysis of 1-st (and may be 2-nd) deviation of your data seems to me is a good way to solve the problem...