Figure 3 shows the frequency response for an ideal bandpass filter. In order to have control over the total audio spectrum, the filters need to be arranged in parallel, as shown in Figure 4. Each filter in the graphic equalizer has the same input. Their job is to only allow a small band of frequencies through. The ideal bandpass filter completely blocks all frequencies outside of the passband.
All practical filters have some ripple in the frequency response.
A diagram of a graphic equalizer with N bands of control. Once the signal passes through the bandpass filters, you can now manipulate each of those frequency bands independently by inserting some kind of gain control. All those sliders on the front of the equalizer are the gain controls in each band. The parallel arrangement of the filters, compared to the series connection with the tone controls, is used to reduce the more harmful effects of the filters. The magnitude frequency responses shown above do not tell us everything about the filter.