@Stefan Schippers How can frequency of below shown transient waveform be calculated in xschem/ngspice ?

xschem does not have advanced signal processing capabilities. You can zoom, add cursors ('a' and 'b' keys) and measure the period (inverse of frequency). More advanced automatic measurement can be done using the ngspice MEAS function. See Ngspice manual, chapter 15.4. You can also use ngspice fft if you want the complete frequency spectrum.