rrdtool is an amazing little tool for storing and graphing time series data, but it’s very idiosyncratic and awkward to use. The commands are confusing and there’s a lot of mystery to the output.

If you run rrdtool graph to produce a graph, and it outputs the cryptic string 0x0, what that means is you forgot to give a command like LINE2 or AREA to actually draw a graph. Normally it prints out a string like 481x154 to tell you the size of the image it drew; 0x0 means it didn’t draw an image at all.