Standing Wave ratio vs frequency in power transmission lines?
In a transmission line, the (voltage) standing wave ratio (or VSWR) is given by

$$\mathrm{VSWR}=\frac{1+\rho}{1-\rho}$$

where \$\rho\$ is the magnitude of the reflection coefficient looking "down" the line. 

In a simple case, \$\rho\$ is non-zero due to an imperfectly matched load at the end of the transmission line, although any other kind of discontinuity in the line (shunt or series elements between segments of line, or changes in the line geometry) also cause reflections and contribute to VSWR.

The reflection at the end of the line is given by

$$\Gamma=\frac{Z_L - Z_0}{Z_L + Z_0}$$

and \$rho\$ is the absolute value of this quantity. \$Z_L\$ is the impedance of the load and \$Z_0\$ is the characteristic impedance of the transmission line. 

If \$\Gamma\$ is positive (\$Z_L &gt; Z_0\$), then this simplifies to

$$\rm{VSWR}=\frac{Z_L}{Z_0}$$

Or, if \$\Gamma\$ is negative (\$Z_L &lt; Z_0\$), then

$$\rm{VSWR}=\frac{Z_0}{Z_L}$$

Now we can answer your question, 


  What is the relationship between Voltage Standing Wave Ratio and frequency in context of transmission lines?


Typically, the VSWR varies with frequency because \$Z_L\$ varies with frequency, although it's also possible that \$Z_0\$ could vary with frequency as well.

To determine the actual relationship \$\mathrm{VSWR}(f)\$, the nature of the load needs to be known so that \$Z_L(f)\$ can be used to determine the VSWR.