Here is a heat map and histogram showing the frequencies of pitches on a standard-tuned guitar. Our ear perceives low frequencies as low pitches (more bass), and high frequencies as high pitches (more treble). I took the frequencies from the 20-fret heat map and plotted them as a histogram, with the bars spaced according to actual frequency rather than using the usual equal spacing. This highlights how the measured spacing in Hz changes for a half step in pitch as you move from low frequencies to high frequencies.

The frequency of a note relative to another simply requires multiplying the original frequency by 1.06^x, where x is the number of half steps between the notes (positive if you are going to a higher pitch, negative if going to a lower pitch). The 1.06 value is a rounding of 2^(1/12). Given this exponential equation, it is clear that half-step increases in pitch will require increasingly large jumps in frequency (Hz). As such, the spacing between the bars on the histogram increases moving from lower to higher frequencies. The histogram shows that the highest incidence of any given pitch is five occurrences, which applies to B3, C4, E4 and F4, where C4 is middle C on a piano.

Data source: http://www.seventhstring.com/resources/notefrequencies.html