Signal ruler is a vertical line which moves over the signal following the mouse pointer. It can be
used to measure parts of the signal or to see the value on certain position.
The current position of the ruler and the corresponding value from the signal at that position are
available in a tooltip next to the ruler in the top of the window:
By turning Harmonic ruler option on in the context menu of FFT-based and signal calculator
windows, the ruler will be shown on the current position of the mouse cursor as well as on all
harmonics of the current frequency (current frequency x2, x3, x4, etc.)