When data must be compared, bars should be adjacent to one another and spaced such that a direct visual comparison can be made without eye movement.
Additional Information:
A horizontal bar chart is illustrated in Figure 1.3. The spacing between bars should be less than the bar width. If many bars are displayed, then spacing may produce an alternating pattern of bright and dark bands that could prove visually disturbing. In this case, it is preferable to arrange the bars contiguously (i.e., without spaces).
Figure 1.3 Example of a horizontal bar chart.