If you're not concerned about precision, you can list the hours of operation vertically and by each hour show icons of people in silhouette. The more people you show, the busier you are.
You could even arrange the icons side by side, which would basically turn them into (coarse-grained) bar charts. That would give users an iconic display of the crowd plus a comparative indicator of what the most crowded times are. Add a cash register and you get images of people waiting in line -- which is self explanatory -- where the length of the line is indicative of how crowded you are.
Here's an example: