Many believe that low volume is a bad sign, however the fact that we've been moving up last few weeks on low, decreasing volume is in fact a very strong sign. It's a sign that very few holders are willing to sell. The investors that recognise that from a long-term perspective the current prices are quite great and hence want to increase their long positions thereby drive up the price more than one would normally expect.
Regarding the graph above: also note the very similar RSI level
My prediction: we'll see 6000+ before the end of May