For your problem the VI "Extract Single Tone Information.vi" (Functions palette-->Analyze-->Waveform Measurements-->Extract Single Tone Information.vi) should be more appropriate.
When you use the peak detection functions, you should be careful since false peaks can be detected due to the noise on your data. The width parameter of the peak detector VI is described in the online help for this VI where it says:
width specifies the number of consecutive data points to use in the quadratic least squares fit. width is coerced to a value greater than or equal to 3. The value should be no more than about 1/2 of the half-width of the peaks/valleys and can be much smaller (but > 2) for noise-free data. Large widths can reduce the apparent amplitude of
peaks and shift the apparent location. For noisy data, this modification is unimportant since the noise obscures the actual peak. Ideally, width should be as as small as possible but must be balanced against the possibility of false peak detection due to noise.
I don't exactly understand what you mean by "..it only counts the current data on screen".
If you wish you can post a typical data set as a text file and I (or somebody else) will write a short analysis VI.
