It is decreasing by half every 4 years, just that graph is just not showing it well for this short timeframe.
The dip up until 2010 was due to there only being one or two miners in the entire network, so the maximum number of Bitcoin wasn't produced in that period. The speed of the entire network couldn't keep up with the minimum of 6 blocks an hour which has lowered the total available Bitcoin in that period, though the number overall is just fine. You can see the sharp decrease in late 2012 when the block reward halving first occurred, which is the start of the downward trend in block reward production.
Currently the network hashrate is increasing so quickly that it's producing more Bitcoin than expected, we'll probably reach the next halving in a little less than the intended 4 year cycle. That's not a negative effect in itself though, it just means the network is increasingly secure month on month. There's a similar hump between June and July of 2011, when GPU mining was publicly introduced.
but doesn't this contradict the previous answer, then? if the rate of growth of bitcoin production is the same regardless of the network power, than the rate of production between jan 2009 and jan 2010 was the one dictated by the algorithm. Why is it, then, that the speed of production INCREASED in jan 2010, when the algorithm should only allow for DECREASES in the speed every 4 years? – user114618 – 2013-12-10T18:10:45.063
when there are very few miners (or not much computational power) in the network, the changes to difficulty which be mucj more volatile of course. If say there are 3 miners each contributing 1/3 of the network and one drops out there will be a large impact. Now if there are 1 million miners and 1 drops out nothing will happen. For a graph showing that the growth of bitcoins in existence has been fairly linear since inception please refer to this: https://blockchain.info/charts/total-bitcoins?timespan=all&showDataPoints=false&daysAverageString=1&show_header=true&scale=0&address=
– user10263 – 2013-12-10T22:56:25.130this is the same link as the one I posted initially and it does show an acceleration in jan 2010 – user114618 – 2013-12-11T09:54:37.507