To be sure about why it appears to be centered around -0.2°C, you'd have to go to the original source of the graph. However, as an educated guess, I would imagine that they are showing the anomaly compared to some recent period, such as 1961-1990 or 1971-2000, as is commonly done. From your Wikimedia Commons link, this figure appears to be original work by a Wikipedia author. The link does not appear to state this, so this is rather sloppy and perhaps illustrates why it's preferable to obtain information from peer reviewed sources.
If the temperature increase stays at +0.4°C, it is not catastrophic. However, considering the greenhouse gases (GHG) we have already pumped into the atmosphere, temperature increase will not stay at +0.4°C. With currently projected increases in GHG emissions, the global average temperature increase may range from anything between +1.5°C and +6.5°C, with some regions exhibiting far larger temperature increases than that. At such scales, the entire climate system might enter a different state, which means many regions will have climates very unlike before. And then I'm not even mentioning the problems associated with sea level rise yet.