I had the same idea, but like always the best answer depends on information we don't have about the original data!

Like whats the maximum timespan, number of stores and expected information gain.

I guess that distinguishing up to 20 stores by color or symbols should be sufficient to represent them all into one 2D chart (Sales x Time). Connecting the points might help:

Like here or even with perspective.

All images found by googling for GD:Graph.