help with bandwidth diagrams for digital transmission
The $x$-axis in the diagrams is normalized frequency (normalized by the data rate). Note that they define $S$, which is the maximum baud rate, and $S_{ave}$, which is the average baud rate. You get the maximum baud rate if your data is such that your signal is changing at a maximum rate. For NRZ-L or Manchester that would be a sequence of alternating $0$s and $1$s. However, on average the baud rate is smaller than that maximum baud rate. This is what is meant by $S_{ave}$, which is less than $S=N/r$.

For baseband signals such as these, the (effective) bandwidth is the maximum frequency above which there are no relevant frequency components. Since the required bandwidth is proportional to the baud rate, and since both Manchester and differential Manchester codes have $r=\frac12$ whereas NRZ-L and NRZ-I have $r=1$, the Manchester codes need approximately twice as much bandwidth as an NRZ scheme.