Position g Center Y
Deletes a vertical offset on your plot by centering it around zero. There are two ways to do this:
using the plot's minimum and maximum Y value: This is probably best for clean (not noisy) plots. It takes the Y value of the highest data point and the Y value of the lowest data point, and shifts the plot so that its mean value is zero: Zero = (Y_{max} + Y_{min}) / 2
using the mean value of one period: Useful when your plot contains spikes, in which case the above method will not work properly. This method tries to determine the mean value of all the Y values of one complete period, and then positions the plot around it. In order to determine the length of one period you first have to draw a rectangle around one. This rectangle doesn't need to be very tight, just try to draw a more or less symmetrical rectangle around one period. The program then will try to determine the actual zero crossings by itself. Of the rectangle only the X values will be used as a starting point to find the zero crossings, so the command can be used to center multiple plots with different offsets at the same time, given that their periods are equal in length. The noisier your plot is, the more erroneous the results of this function can be, so take a bit of care to visually check if the centering was done properly. You can always try to smooth a copy of your plot, center it, and then shift the original plot with the same factor which was used when centering the smoothed plot. This can be easily done, because:
The shift factor will be stored in the Shift Y default value, so that you can later shift other plots with the same factor, when desired.