Slipped sideways

Posted by Paul on March 29, 2008 at 15:56:

I am not sure I understand the following sentence from a book I'm reading: "In 1834, 40 per cent more tea was shipped to Britain than the year before, and of course the price slipped sideways."

Does this phrase mean that prices fell, but because consumption increased, the overall cost of the tea was the same? Or does this mean that the price originally dropped (because there was 40% more), but then, say, by the end of the year, was back to its normal price?

The book is about the Opium Wars so the rest of the paragraph really doesn't give me context on the price of tea--that was just thrown in.