Since this is just volume measured by number of bitcoins exchanged, wouldn't it be more accurate to scale it by the exchange rate of your primary currency (ie. the product of the two graphs)? That seems like it would more closely represent the value being exchanged.
This seems to support the conclusion that transaction volume (in USD) is growing significantly, although the trend is confounded by large price movements.
Edit:
Looks like they have this graph as well: https://blockchain.info/charts/estimated-transaction-volume-... (log scale).
This seems to support the conclusion that transaction volume (in USD) is growing significantly, although the trend is confounded by large price movements.