Now, look at the tangent lines on the right side...
 

The slopes not only tell us that revenue is decreasing...  But, those values are getting smaller!  This is NOT a good thing when revenue is involved.

OK, let's look at one of those valleys:

Look at the tangent lines on the left...
 

Things are bad...  But, although those slopes are negative, they're getting bigger.

This could mean that a much needed turnaround in revenue is coming!  Look at the bottom...  There's one of those horizontal tangent lines again!